init script: support DAEMON_ARGS

Support additional command line parameters by sourcing
/etc/default/pdns and taking DAEMON_ARGS from there.
init script: ensure socket dir exists

This is an old change in Debian that is apparently necessary on some
(newer) systems where the socket dir might vanish on reboot.
Merge pull request #1724 from zeha/f/fix-1149

pdns init script improvements, fixes #1149, closes #1721 (thanks @tuxis-ie as well!)