gpg: Print warning when rejecting weak digests

* g10/misc.c (print_md5_rejected_note): Rename to ..
(print_digest_rejected_note): this.  Parameterize function to take an
enum gcry_md_algos.
* g10/sig-check.c: Use print_digest_rejected_note() when rejecting
signatures.

76afaed65e3b0ddfa4923cb577ada43217dd4b18 allowed extra --weak-digests,
but removed the one call to print_md5_rejected_note(). This replaces
and generalizes that warning.

• Signed-Off-By: Daniel Kahn Gillmor <dkg@fifthhorseman.net>