procd: support pidfile writing.

procd from revision b12bb150ed38a4409bef5127c77b060ee616b860 supports
writing a pidfile.  This adds support for setting that parameter with
standard init script hooks:

   procd_set_param pidfile /var/run/someprocess.pid

Signed-off-by: Karl Palsson <karlp@etactica.com>

SVN-Revision: 48984