pfctl: cache getprotobynumber results

As for example pfctl -ss keeps calling it, it saves a lot of overhead
from elided parsing of /etc/nsswitch.conf and /etc/protocols.

Sample result when running a pre-nvlist binary with nfs root and dumping
7 mln states:
before: 24.817u 62.993s 1:28.52 99.1%
after: 8.064u 1.117s 0:18.87 48.5%

Idea by Jim Thompson

Reviewed by: kp
Sponsored by: Rubicon Communications, LLC ("Netgate")