gpgv validates a signature against a file.
gpgv [-v] KEY SIG CONTENT
It prints "OK\n" to stdout if the check succeeds and exits with 0. It prints an error message and exits with non-0 otherwise. The openpgp package ReadKeyRing function does not completely implement RFC4880 in that it can't use a PublicSigningKey with 0 signatures. We use one from Eric Grosse instead.