For now, DragonFly and FreeBSD use the libc version, it is not reentrant,
but thread-safe. NetBSD 3.0+ and Darwin 8.0+ use libresolv from base
(the BIND9 resolver), all other fall back to net/bind9. Feel free to add
your favorite platform if it has a thread-safe resolver in base.
Modify mail/libspf-alf, mail/milter-greylist, mail/spamdyke and
net/nocol accordingly. Testing on !DragonFly and feedback from tron@
