bsdinstall: Ignore -p[0-9]+ in determining BRANCH

The patch level is not part of the branch per se and should not be
used in constructing the FreeBSD-base.conf file used by bsdinstall.

Approved by: re (cperciva)
MFC after: 1 day

(cherry picked from commit d45816f369eb0099fd274fee23a3bf137ee794b1)
(cherry picked from commit 56275b7b06726aa530c1bcab176d225d5d4b7893)