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=16851Closes#16852
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