verify-tag: move tag verification code to tag.c

verify-tag: move tag verification code to tag.c

The PGP verification routine for tags could be accessed by other modules
that require to do so.

Publish the verify_tag function in tag.c and rename it to gpg_verify_tag
so it does not conflict with builtin/mktag's static function.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Santiago Torres <santiago@nyu.edu>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>