cap_dns, cap_net: fix host and service buffer handling

If a malicious casper process sent a host or service string that was
too long, cap_getnameinfo would overrun the caller's buffer by one byte.

The backends for this function needlessly allocated one extra byte
for these buffers. This was harmless, but could be confusing to readers.

Reported by: Coverity (an internal run at Dell)
Reviewed by: oshogbo, emaste
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D39347