curlx: move Curl_strerror, use in src and tests, ban strerror globally

Also:
- tests/server: replace local `sstrerror()` with `curlx_strerror()`.
- tests/server: show the error code next to the string, where missing.
- curlx: use `curl_msnprintf()` when building for src and tests.
  (units was already using it.)
- lib: drop unused includes found along the way.
- curlx_strerror(): avoid compiler warning (and another similar one):
  ```
  In file included from servers.c:14:
  ../../lib/../../lib/curlx/strerr.c: In function ‘curlx_strerror’:
  ../../lib/../../lib/curlx/strerr.c:328:32: error: ‘snprintf’ output may be truncated before the last format character [-Werror=format-truncation=]
    328 |       SNPRINTF(buf, buflen, "%s", msg);
        |                                ^
  ../../lib/../../lib/curlx/strerr.c:47:18: note: ‘snprintf’ output 1 or more bytes (assuming 2) into a destination of size 1
     47 | #define SNPRINTF snprintf
        |                  ^
  ../../lib/../../lib/curlx/strerr.c:328:7: note: in expansion of macro ‘SNPRINTF’
    328 |       SNPRINTF(buf, buflen, "%s", msg);
        |       ^~~~~~~~
  ```

Follow-up to 45438c8d6f #18823

Closes #18840

src/tool_formparse.c

@@ -222,8 +222,9 @@ size_t tool_mime_stdin_read(char *buffer,
       /* Read from stdin. */
       nitems = fread(buffer, 1, nitems, stdin);
       if(ferror(stdin)) {
+        char errbuf[STRERROR_LEN];
         /* Show error only once. */
-        warnf("stdin: %s", strerror(errno));
+        warnf("stdin: %s", curlx_strerror(errno, errbuf, sizeof(errbuf)));
         return CURL_READFUNC_ABORT;
       }
     }
@@ -444,8 +445,9 @@ static int read_field_headers(const char *filename, FILE *fp,
     switch(c) {
     case EOF:
       if(ferror(fp)) {
+        char errbuf[STRERROR_LEN];
         errorf("Header file %s read error: %s", filename,
-               strerror(errno));
+               curlx_strerror(errno, errbuf, sizeof(errbuf)));
         return -1;
       }
       return 0;    /* Done. */
@@ -564,9 +566,11 @@ static int get_param_part(char endchar,
         sep = *p;
         *endpos = '\0';
         fp = curlx_fopen(hdrfile, FOPEN_READTEXT);
-        if(!fp)
+        if(!fp) {
+          char errbuf[STRERROR_LEN];
           warnf("Cannot read from %s: %s", hdrfile,
-                strerror(errno));
+                curlx_strerror(errno, errbuf, sizeof(errbuf)));
+        }
         else {
           int i = read_field_headers(hdrfile, fp, &headers);