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/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 */