Add pidfile support to ggated(8)

Add pidfile support to ggated(8)

The tests will manipulate the system daemon today, which can cause undesired
service interruption when the tests are run.

This change allows the geom_gate tests to be run with an arbitrary ggated(8)
daemon / geom_gate(4) device pairing.

Other changes:
- Sort #includes
- Use a more common idiom for parsing options with getopt(3)

Differential Revision: https://reviews.freebsd.org/D4836
MFC after: 2 weeks
Reviewed by: bjk (manpages), pjd (maintainer timeout)
Sponsored by: EMC / Isilon Storage Division