crypt: Deduplicate get_public_key().

The openssl and grypt implementations of this function share quite
some code. This patch factors out the common code into the new
decode_ssh_key() helper of crypt_common.c. Both implementations are
changed to call the new helper.

As a side effect of this change, all callers of is_ssh_rsa_key()
and check_ssh_key_header() now reside in crypt_common.c, so we can
make these two functions static and remove their declarations from
crypt_backend.h.