as pointed out by Bjoern Labitzke <hermit@cs.tu-berlin.de> in PR/7359.
While I'm at it, also correct the behaviour when invoked as "pgp5"
(give usage, not unhelpful 'invoked with unknown symbolic link').
Based on pr 6684 by Martin J. Laubach with heavy refining/fixing by myself.
XXX This package needs still some tweaking to work with rsaref.
XXX Maybe some US citizen developer can take over?
XXX At least its usefull to non-US ppl.