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)

https://github.com/eggheads/eggdrop/blob/d317ac228eb6682fdd6386007e09d627efd618ca/src/dns.c#L620 after:

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