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)