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.