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

lib/strerror.h

@@ -24,11 +24,6 @@
  *
  ***************************************************************************/
 
-#include "urldata.h"
-
-#define STRERROR_LEN 256 /* a suitable length */
-
-const char *Curl_strerror(int err, char *buf, size_t buflen);
 #ifdef USE_WINDOWS_SSPI
 const char *Curl_sspi_strerror(int err, char *buf, size_t buflen);
 #endif