Fix tdns leak and init attr only once

[michaelortmann]

Found by: https://github.com/michaelortmann
Patch by: https://github.com/michaelortmann
Fixes:

One-line summary:
Fix tdns leak and init attr only once

Additional description (if needed):
Also cleaned up code structure, moved tdns init code from main.c to dns.c:init_tdns()

Test cases demonstrating functionality (if applicable):
before:

$ valgrind --leak-check=full ./eggdrop -t BotA.conf 2>&1|grep dns.c
.tcl dnslookup google.com foo
.die
==11075==    by 0x24E202: core_dns_ipbyhost (dns.c:620)
==11075==    by 0x24E973: tcl_dnsipbyhost (dns.c:369)
==11075==    by 0x24E640: tcl_dnslookup (dns.c:739)

eggdrop/src/dns.c

Line 620 in d317ac2

if (pthread_attr_init(&attr)) {

after:

$ valgrind --leak-check=full ./eggdrop -t BotA.conf 2>&1|grep dns.c
.tcl dnslookup google.com foo
.die