serv: add support for foregrounding, running with and without PID file,
including custom PID filename.
test: verify Finit can start & monitor services that:
1) Fork and creates a PID file in a known location
2) Don't fork and don't create a PID file, but Finit does
3) Don't fork but create a PID file
4) Don't fork and create custom named PID file
Note: Finit cannot support a service that forks and doesn't create a PID
file. This combination is impossible to support without tracking
all processes created in /proc -- which Finit does not do atm.
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
Joachim Wiberg