Allow user associated with process to stop it.

[AlexEshoo]

As explained in #20 the process stored in /var/run/pid_file is the PID for the command sudo -U <user> <cmd> which causes an error when the user attempts to stop the process since kill <pid> is not allowed because the pid is owned by root.

pgrep -U "$user" -xf "$cmd" captures the pid for the actual running script which is owned by $user, therefore allowing them to successfully stop the service without sudo.