cap_dns, cap_net: fix host and service buffer handling

[vangyzen]

Summary

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
MFC after: 1 week
Sponsored by: Dell EMC Isilon

Test Plan

$ kyua debug net_test:capnet__getnameinfo_overflow
net_test:capnet__getnameinfo_overflow  ->  passed

[emaste]

please add the CID to the commit message

[emaste]

(unless this is not from Coverity Scan's FreeBSD report)

[oshogbo]

LGTM.
Nice catch.

[vangyzen]

please add the CID to the commit message
(unless this is not from Coverity Scan's FreeBSD report)

It's from an internal run at Dell (Isilon). Oddly enough, I can't find this issue in the public FreeBSD report.

Thanks for the reviews, folks.