keygen: Fix curve key generation

zmq_curve_keypair() generates keys with z85 encoding, but
zcert_new_from() expected decoded keys. Therefore, decode keys
with zmq_z85_decode() first before passing to zcert_new_from().

Yes, this seems like unnecessary work, but we use zmq_curve_keypair()
to ensure that zeroed keys are not silently generated in zcert_new(),
and this operation should only occur rarely, during 'flux keygen' so
we are not too worried about speed.
remove references to --curve in flux-keygen

flux-keygen didn't actually export -c, --curve option, and curve
is the default anyway, so remove all references to the 'curve' option
in the code to avoid confusion.
security: fix crash in genpasswd

genpasswd() tried to free 'passwd' as returned from zuuid_str(), but
the return value from zuuid_str() is not allocated with malloc but
is part of the zuuid_t object itself (and thus will be freed with
zuuid_destroy()).

This fixes a crash in flux-keygen --plain.
Merge pull request #12 from grondo/zcert_curve_new-fix

More keygen fixes