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