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2021-07-05T12:02:56.399295Z
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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")

(cherry picked from commit 858937bea4599d254a97ee6321683f8629604e15)

Git Hash:   98fd94dc83319db1b6bfc9f741245eefb0af7911
Git Author: mjg@FreeBSD.org
END