lpd: Tighten permissions on /var/run/printer

Exclude group read/write permissions as well. Otherwise, group wheel can
submit things w/o the normal accounting. While group wheel is generally
trusted on the machine, submitting jobs w/o checks is not one of the
functions we document for that group.

PR: 17289
Differential Revision: https://reviews.freebsd.org/D47040

(cherry picked from commit 6cde8f3ef7cd060be39d78055eaf982b06b19a3b)