Create default pid file on startup.

After this commit here is how pidfile name is set (each step have
precedence over the earlier one):

1. The default pidfile name is $TMPDIR/oscam.pid (/tmp/.oscam/oscam.pid)
2. If pidfile= is set in oscam.conf [global] then it is used instead.
3. If --pidfile (-B) command line parameter is set then it is used instead.

git-svn-id: http://streamboard.de.vu/svn/oscam/trunk@8189 4b0bc96b-bc66-0410-9d44-ebda105a78c1