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