tests/server: make the signal handler signal-safe

[vszakats]

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 e5bb88b tool: use our own stderr variable #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