Race fix: tdns: strerror() -> strerror_r()

[michaelortmann]

Found by: thommey and michaelortmann
Patch by: michaelortmann
Fixes:

One-line summary:
Fix locking / race for tdns error logging

Additional description (if needed):
Non main threads like tdns must use strerror_r() instead of strerror().

Test cases demonstrating functionality (if applicable):

valgrind --tool=helgrind ./eggdrop -nt BotA.conf
[...]
.tcl dnslookup google.de foo

Without this patch applied, valgrind will report race

[thommey]

       int strerror_r(int errnum, char buf[.buflen], size_t buflen);
                      /* XSI-compliant */
       char *strerror_r(int errnum, char buf[.buflen], size_t buflen);
                      /* GNU-specific */
       strerror_r():
           The XSI-compliant version is provided if:
               (_POSIX_C_SOURCE >= 200112L) && ! _GNU_SOURCE
           Otherwise, the GNU-specific version is provided.

I have no words, but both work in our context, LGTM