pfctl: Allow a semicolon (;) as a comment

To make parsing of, for example, Spamhaus' drop.txt and similar
files that contains semicolons as comments, allow them also
in file-based tables.

Reviewed by: kp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D46088

(cherry picked from commit a8a95277363be2c92b3c06bd9cd1a32d1c6c6ecd)