tests/server: make the signal handler signal-safe

Before this patch the signal handler called `logmsg()` which in turn
called `printf()` variants (internal implementations), and `FILE *`
functions, `localtime()`. Some of these called `malloc`/`free`, which
isn't supported in s signal handler. Replace them with `write` calls,
losing some logging functionality.

Also:
- De-dupe and move `STD*_FILENO` macros to `lib/curl_setup.h`. Revert
  the `src` definition to point to `stderr`, instead of `tool_stderr`.
  Follow-up to e5bb88b8f8 #11958

POSIX specs with list of functions allowed in a signal handler:
2004: https://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_04.html#tag_02_04_03
2017: https://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_04_03
2024: https://pubs.opengroup.org/onlinepubs/9799919799/functions/V2_chap02.html#tag_16_04_03

Linux CI run with the thread sanitizer going crazy when
hitting the signal handler in test 1238 and 1242 (TFTP):
```
WARNING: ThreadSanitizer: signal-unsafe call inside of a signal (pid=12582)
    #0 malloc <null> (servers+0x5ed70)
    #1 _IO_file_doallocate <null> (libc.so.6+0x851b4)
    #2 formatf /home/runner/work/curl/curl/bld/tests/server/../../lib/../../lib/mprintf.c:886:9 (servers+0xdff77)
[...]
WARNING: ThreadSanitizer: signal-unsafe call inside of a signal (pid=12582)
    #0 free <null> (servers+0x5f453)
    #1 fclose <null> (libc.so.6+0x8532f)
    #2 logmsg /home/runner/work/curl/curl/bld/tests/server/../../../tests/server/util.c:134:5 (servers+0xe684d)
```
Ref: https://github.com/curl/curl/actions/runs/14118903372/job/39555309490?pr=16851

Closes #16852

src/tool_main.h

@@ -33,16 +33,4 @@
 #define MAX_PARALLEL 300 /* conservative */
 #define PARALLEL_DEFAULT 50
 
-#ifndef STDIN_FILENO
-#  define STDIN_FILENO  fileno(stdin)
-#endif
-
-#ifndef STDOUT_FILENO
-#  define STDOUT_FILENO  fileno(stdout)
-#endif
-
-#ifndef STDERR_FILENO
-#  define STDERR_FILENO  fileno(tool_stderr)
-#endif
-
 #endif /* HEADER_CURL_TOOL_MAIN_H */

src/tool_setup.h

@@ -98,8 +98,6 @@ extern bool tool_term_has_bold;
 #  define _get_osfhandle(fd) (fd)
 #  undef _getch
 #  define _getch() 0
-#  undef STDIN_FILENO
-#  define STDIN_FILENO 0
 #endif
 
 #ifndef HAVE_FTRUNCATE