authpf(8) read_config() should chop off trailing white space

if administrator mistakenly types into configuration file

anchor=authpf_test

where 'authpf_test' is followed by white space, the authpf(8)
is going to use anchor 'authpf_test ' instead of the 'authpf_test'
which is defined in pf.conf(5) as 'anchor authpf_test/*'

issue kindly reported and patch submitted by

Avinash Duduskar <avinash.duduskar (_at_) gmail (_dot_) com>

OK sashan@

PR: 296958
MFC after: 1 week
Obtained from: OpenBSD, sashan <sashan@openbsd.org>, 2d12a8e44d
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 04f25ef716f74d6bb7941750091c6cb4b51d0b4d)