Detect invalid .nf option and fail gracefully

(reported by kdweeks:
   http://www.pgdp.net/phpBB2/viewtopic.php?p=1000257#1000257 )

(Note: fixes were needed in several other spots than the one kd reported.)
Merge pull request #60 from wf49670/develop

Detect invalid .nf option and fail gracefully. Corrections by wf49670. Thank you Walt.