systemd-notify: Always pass a valid pid to sd_pid_notify

      If the option --pid was used, take the pid from this option, unless take
the parend pid. Using 0 as pid (ucred of systemd-notify) will result 99% of the
time in a failure with this error: "Cannot find unit for notify message of PID"

Shouldn't we use always the ppid, since the MAINPID is something else ?

Signed-off-by: Benjamin Robin <dev@benjarobin.fr>