libc/resolve: Fix broken EDNS option

libc/resolve: Fix broken EDNS option

EDNS option broken with unbound.  struct querybuf in net/gethnamaddr.c
defines MAXPACKET to 64K.  This in turn gets passed down until it
reached res_nopt(..., answer, anslen), where the size of the buffer
must fit in 16 bits.  Unfortunately we end up being one more than the
max so we end up sending a 0 as the size and unbound does not like
that. Instead we clip now to 64K - 1, and everyone is happy.

Obtained-from: NetBSD
DragonFly-bug: http://bugs.dragonflybsd.org/issues/2846