udev-173: don't hardcode path to pidof

* pidof lives in /bin, search it in $PATH.
* Assume pidof's presence.
* Remove -e shell option, because pidof may return nonzero.

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>