build: fix some -Wsign-conversion/-Warith-conversion warnings

- enable `-Wsign-conversion` warnings, but also setting them to not
  raise errors.
- fix `-Warith-conversion` warnings seen in CI.
  These are triggered by `-Wsign-converion` and causing errors unless
  explicitly silenced. It makes more sense to fix them, there just a few
  of them.
- fix some `-Wsign-conversion` warnings.
- hide `-Wsign-conversion` warnings with a `#pragma`.
- add macro `CURL_WARN_SIGN_CONVERSION` to unhide them on a per-build
  basis.
- update a CI job to unhide them with the above macro:
  https://github.com/curl/curl/actions/workflows/linux.yml -> OpenSSL -O3

Closes #12492

lib/curl_setup.h

@@ -28,6 +28,13 @@
 #define CURL_NO_OLDIES
 #endif
 
+/* FIXME: Delete this once the warnings have been fixed. */
+#if !defined(CURL_WARN_SIGN_CONVERSION)
+#ifdef __GNUC__
+#pragma GCC diagnostic ignored "-Wsign-conversion"
+#endif
+#endif
+
 /* Set default _WIN32_WINNT */
 #ifdef __MINGW32__
 #include <_mingw.h>

lib/doh.c

@@ -444,7 +444,7 @@ static DOHcode skipqname(const unsigned char *doh, size_t dohlen,
       return DOH_DNS_BAD_LABEL;
     if(dohlen < (*indexp + 1 + length))
       return DOH_DNS_OUT_OF_RANGE;
-    *indexp += 1 + length;
+    *indexp += (unsigned int)(1 + length);
   } while(length);
   return DOH_OK;
 }
@@ -456,14 +456,15 @@ static unsigned short get16bit(const unsigned char *doh, int index)
 
 static unsigned int get32bit(const unsigned char *doh, int index)
 {
-   /* make clang and gcc optimize this to bswap by incrementing
-      the pointer first. */
-   doh += index;
+  /* make clang and gcc optimize this to bswap by incrementing
+     the pointer first. */
+  doh += index;
 
-   /* avoid undefined behavior by casting to unsigned before shifting
-      24 bits, possibly into the sign bit. codegen is same, but
-      ub sanitizer won't be upset */
-  return ( (unsigned)doh[0] << 24) | (doh[1] << 16) |(doh[2] << 8) | doh[3];
+  /* avoid undefined behavior by casting to unsigned before shifting
+     24 bits, possibly into the sign bit. codegen is same, but
+     ub sanitizer won't be upset */
+  return ((unsigned)doh[0] << 24) | ((unsigned)doh[1] << 16) |
+         ((unsigned)doh[2] << 8) | doh[3];
 }
 
 static DOHcode store_a(const unsigned char *doh, int index, struct dohentry *d)

lib/inet_pton.c

@@ -112,7 +112,8 @@ inet_pton4(const char *src, unsigned char *dst)
 
     pch = strchr(digits, ch);
     if(pch) {
-      unsigned int val = *tp * 10 + (unsigned int)(pch - digits);
+      unsigned int val = (unsigned int)(*tp * 10) +
+                         (unsigned int)(pch - digits);
 
       if(saw_digit && *tp == 0)
         return (0);

lib/mprintf.c

@@ -1010,7 +1010,7 @@ number:
 static int addbyter(int output, FILE *data)
 {
   struct nsprintf *infop = (struct nsprintf *)data;
-  unsigned char outc = (unsigned char)output;
+  char outc = (char)output;
 
   if(infop->length < infop->max) {
     /* only do this if we haven't reached max length yet */

lib/share.c

@@ -133,13 +133,13 @@ curl_share_setopt(struct Curl_share *share, CURLSHoption option, ...)
       res = CURLSHE_BAD_OPTION;
     }
     if(!res)
-      share->specifier |= (1<<type);
+      share->specifier |= (unsigned int)(1<<type);
     break;
 
   case CURLSHOPT_UNSHARE:
     /* this is a type this share will no longer share */
     type = va_arg(param, int);
-    share->specifier &= ~(1<<type);
+    share->specifier &= ~(unsigned int)(1<<type);
     switch(type) {
     case CURL_LOCK_DATA_DNS:
       break;
@@ -264,7 +264,7 @@ Curl_share_lock(struct Curl_easy *data, curl_lock_data type,
   if(!share)
     return CURLSHE_INVALID;
 
-  if(share->specifier & (1<<type)) {
+  if(share->specifier & (unsigned int)(1<<type)) {
     if(share->lockfunc) /* only call this if set! */
       share->lockfunc(data, type, accesstype, share->clientdata);
   }
@@ -281,7 +281,7 @@ Curl_share_unlock(struct Curl_easy *data, curl_lock_data type)
   if(!share)
     return CURLSHE_INVALID;
 
-  if(share->specifier & (1<<type)) {
+  if(share->specifier & (unsigned int)(1<<type)) {
     if(share->unlockfunc) /* only call this if set! */
       share->unlockfunc (data, type, share->clientdata);
   }

lib/vauth/krb5_gssapi.c

@@ -226,7 +226,8 @@ CURLcode Curl_auth_create_gssapi_security_message(struct Curl_easy *data,
   /* Extract the security layer and the maximum message size */
   indata = output_token.value;
   sec_layer = indata[0];
-  max_size = (indata[1] << 16) | (indata[2] << 8) | indata[3];
+  max_size = ((unsigned int)indata[1] << 16) |
+             ((unsigned int)indata[2] << 8) | indata[3];
 
   /* Free the challenge as it is not required anymore */
   gss_release_buffer(&unused_status, &output_token);

lib/vauth/krb5_sspi.c

@@ -319,7 +319,8 @@ CURLcode Curl_auth_create_gssapi_security_message(struct Curl_easy *data,
   /* Extract the security layer and the maximum message size */
   indata = input_buf[1].pvBuffer;
   sec_layer = indata[0];
-  max_size = (indata[1] << 16) | (indata[2] << 8) | indata[3];
+  max_size = ((unsigned long)indata[1] << 16) |
+             ((unsigned long)indata[2] << 8) | indata[3];
 
   /* Free the challenge as it is not required anymore */
   s_pSecFn->FreeContextBuffer(input_buf[1].pvBuffer);