Init script: Set variables before sourcing sysconfig.

Doing this after setting the variables allows sysconfig settings to
override the configure/install-time defaults.
Init script: Whitespace and formatting cleanup.
Init script: Don't try to start if already running.

Running start with an instance of nagios already running would result
in the running instance's command pipe being removed. Test that the
lock file does not hold the PID of a running process before attempting
to start a new instance.

Also remove some unused arguments from status_nagios calls.
Init script: Quote kill's signal specification.

This gives a little bit less confusing error message if someone were to
invoke killproc_nagios without specifying a signal to send.