Added new global configuration directive shutdown_script.

[bfg]

This patch adds new global configuration directive "shutdown_script". Using this directive one can, for example, easily shutdown any services started by notitfy_* directive scripts when daemon is shut down.

Example: Run /bin/true on daemon shutdown.
shutdown_script "/bin/true arg1 arg2"

[acassen]

shutdown_script MUST not be called from signal handler. Would be more appropriated to run script at stop_keepalived(void) level so that waitpid (VRRP & CHECK) have completed and child are stopped.

You can even imagine to register multiple shutdown_script for each subsystem to be called in each subsystem: I mean :
shutdown_script_global
shutdown_script_check
shutdown_script_vrrp

Then all subsystems will have the ability to run script in their own "stop_" directive.