Improve detection of CMS objects.

* configure.ac: Detect gpgme_data_identify.
* src/filetype.c (is_cms_data, is_cms_file): Use gpgme_data_identify
is available.

The gpgme_data_identify function has the advantage that it also
detects X.509 certificates. This offers way to detect certificate
files and import them. As of now the import function does only work
with PGP.