Add support for saving pid file.

This adds new setting in [global] named "pidfile". This setting
holds filename in which pid of the oscam process would be written
on start/restart.

Example oscam.conf:

[global]
pidfile = /var/run/oscam.pid

This closes ticket #3079.

git-svn-id: http://streamboard.de.vu/svn/oscam/trunk@8172 4b0bc96b-bc66-0410-9d44-ebda105a78c1