lpd: Add -F flag to prevent daemonizing

This is necessary for use with supervision, e.g. runit.

I chose -F simply because that is what the folks at LPRng use.

Approved by: pfg, gad, ngie
Differential Revision: https://reviews.freebsd.org/D29566

(cherry picked from commit 3c6366067036d4573528309a0a4d3b52e2a76ae7)