Fix dhcrelay_monitor pid check

test -e "/proc/$pid"

Is not good enough.

In case of non-existent pidfile we just ensure /proc/ exists which
is always True. It's better to check /proc/$pid/fd

Closes-bug: #1346093
Change-Id: Ie8741137266c23a903c20898e5a6143ff2b53502
(cherry picked from commit cee07514e0d2097386df5b0983353f1cba1ad66b)