pub/rsa-gen.c, progs/key.c: Overhaul RSA key generation.

pub/rsa-gen.c, progs/key.c: Overhaul RSA key generation.

Rewrite the key-generation code from scratch.  The new version seems
simpler to me, and allows the caller to choose the public exponent.  It
also retries repeatedly until it finds acceptable values unless told to
stop within a finite number of steps.

Add an option to `key' to allow the user to select a different
exponent.  Recommend e = 3 in the manpage.