Add MD2 message digest implementation

* cipher/md2.c: New.
* cipher/md.c (digest_list): add _gcry_digest_spec_md2.
* tests/basic.c (check_digests): add MD2 test vectors.
* configure.ac (default_digests): disable md2 by default.
--
Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>

Some minor indentation fixes by wk.
tests: Print diagnostics for skipped tests.

* tests/basic.c (show_note): New.
(show_md_not_available):
(show_old_hmac_not_available):
(show_mac_not_available):
(check_digests): Remove USE_foo cpp tests from the test table.  Call
show_md_not_available if algo is not available.
(check_hmac): Likewise.
(check_mac): Likewise.

Signed-off-by: Werner Koch <wk@gnupg.org>