clang-tidy: enable bugprone-signed-char-misuse, fix fallouts

Examples:
```
lib/vtls/openssl.c:2585:18: warning: 'signed char' to 'int' conversion; consider casting to 'unsigned char' first. [bugprone-signed-char-misuse]
 2585 |       msg_type = *(const char *)buf;
lib/vtls/openssl.c:2593:18: warning: 'signed char' to 'int' conversion; consider casting to 'unsigned char' first. [bugprone-signed-char-misuse]
 2593 |       msg_type = *(const char *)buf;
tests/server/mqttd.c:514:10: warning: comparison between 'signed char' and 'unsigned char' [bugprone-signed-char-misuse]
  514 |       if(passwd_flag == (char)(conn_flags & passwd_flag)) {
tests/server/tftpd.c:362:13: warning: 'signed char' to 'int' conversion; consider casting to 'unsigned char' first. [bugprone-signed-char-misuse]
  362 |         c = test->rptr[0];
tests/server/tftpd.c:454:9: warning: 'signed char' to 'int' conversion; consider casting to 'unsigned char' first. [bugprone-signed-char-misuse]
  454 |     c = *p++;                     /* pick up a character */
src/tool_urlglob.c:272:46: warning: 'signed char' to 'int' conversion; consider casting to 'unsigned char' first. [bugprone-signed-char-misuse]
  272 |     pat->c.ascii.letter = pat->c.ascii.min = min_c;
src/tool_urlglob.c:273:24: warning: 'signed char' to 'int' conversion; consider casting to 'unsigned char' first. [bugprone-signed-char-misuse]
  273 |     pat->c.ascii.max = max_c;
tests/libtest/cli_h2_pausing.c:164:23: warning: suspicious usage of 'sizeof()' on an expression of pointer type [bugprone-sizeof-expression]
  164 |   memset(&resolve, 0, sizeof(resolve));
tests/libtest/cli_upload_pausing.c:158:23: warning: suspicious usage of 'sizeof()' on an expression of pointer type [bugprone-sizeof-expression]
  158 |   memset(&resolve, 0, sizeof(resolve));
tests/libtest/first.c:86:15: warning: 'signed char' to 'int' conversion; consider casting to 'unsigned char' first. [bugprone-signed-char-misuse]
   86 |     coptopt = arg[optpos];
```

Also:
- tests/server/mqttd: drop a redundant and a wrongly signed cast.

Ref: https://clang.llvm.org/extra/clang-tidy/checks/bugprone/signed-char-misuse.html

Closes #20654

.clang-tidy.yml

@@ -13,6 +13,7 @@ Checks:
   - -clang-diagnostic-nullability-extension
   - bugprone-macro-parentheses
   - bugprone-redundant-branch-condition
+  - bugprone-signed-char-misuse
   - bugprone-suspicious-realloc-usage
   - misc-const-correctness
   - misc-header-include-cycle