Merge branch '2.3.x'

* 2.3.x:
  Release 2.3.1.
  pub/bbs-gen.c, pub/rsa-gen.c: Remove the lower-bounding on q.
  math/strongprime.c: Clamp the starting point.
  math/strongprime.c: Reduce failures by adding some more slop bits.
  progs/catcrypt.c, progs/cc-sig.c: Compare MAC tags in constant time.
  progs/cc-sig.c: Initialize hash context properly for RSA-PSS.
  progs/cc-sig.c: Don't destroy an RSA context just after building it.
  math/g-bin.c, math/g-prime.c: Fix type incompatibility.
  math/g-*.c: Group implementations include `group.h' via `group-guts.h'.
  key/key-io.c: Produce valid key lines for empty keys.
  key/key-io.c: Fix segfault opening `KOPEN_READ | KOPEN_NOFILE' key files.

Conflicts:
	math/group-guts.h (trivial)
	progs/catcrypt.c (already picked up)