gpg: Print revocation certificate details when showing with-colons.

* g10/import.c (import_revoke_cert): add options argument, and print
colon-delimited output for revocation certificate as requested.

I looked into trying to make this work with one of the functions in
g10/keylist.c, but i saw nothing that will accept a revocation
certificate on its own, so i'm replicating the functionality directly
in g10/import.c. This is a bit unfortunate because the code for
describing a revocation cert now exists in two separate places, but
refactoring both list_keyblock_print() and list_keyblock_colon() in
g10/keylist.c seems like a much heavier lift.

• GnuPG-Bug-id: T4018
• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>