mail/thunderbird: use libc for res_* functions

thunderbird's javascript code needs more than basic DNS resolution
(e.g. SRV, TXT, MX lookups) - more than javascript natively has
(welcome to the web) and builds it's own bindings for libc to use
the resolver(3) interface. Unlike linux, where the resolver routines
live in libresolv, FreeBSD has these routines in libc.
I'm referencing libc.so.7 directly, as this is enough to make this
code work on 13/amd64 (others should work too) - the official way
of using libc.so would be much more work (and reimplementing parts
of the dynamic linker in javascript takes the cake, or something).

This is enough to make adding and using new CalDAV calendars possible;
other problems might linger and possibly require a better solution.