In order to make MSAN happy:

    ==2200945==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x596f3b3ed246 in curlx_strtoofft [...]/libcurl/src/lib/strtoofft.c:239:11
    #1 0x596f3b402156 in Curl_httpchunk_read [...]/libcurl/src/lib/http_chunks.c:149:12
    curl#2 0x596f3b348550 in readwrite_data [...]/libcurl/src/lib/transfer.c:607:11
    [...]

    ==2202041==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x5a3fab66a72a in Curl_parse_port [...]/libcurl/src/lib/urlapi.c:547:8
    #1 0x5a3fab650645 in parse_authority [...]/libcurl/src/lib/urlapi.c:796:12
    curl#2 0x5a3fab6740f6 in parseurl [...]/libcurl/src/lib/urlapi.c:1176:16
    curl#3 0x5a3fab664fc5 in parseurl_and_replace [...]/libcurl/src/lib/urlapi.c:1342:12
    [...]

    ==2202320==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x569076a0d6b0 in ipv4_normalize [...]/libcurl/src/lib/urlapi.c:683:12
    #1 0x5690769f2820 in parse_authority [...]/libcurl/src/lib/urlapi.c:803:10
    curl#2 0x569076a160f6 in parseurl [...]/libcurl/src/lib/urlapi.c:1176:16
    curl#3 0x569076a06fc5 in parseurl_and_replace [...]/libcurl/src/lib/urlapi.c:1342:12
    [...]

Signed-off-by: Louis Solofrizzo <lsolofrizzo@scaleway.com>
Closes curl#12995

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 curl#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)
    curl#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)
    curl#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 curl#16852