sm: Support AES-GCM decryption.

* tests/cms/samplemsgs/: Add sample messages.
* sm/gpgsm.c (main): Use gpgrt_fcancel on decryption error.
* sm/decrypt.c (decrypt_gcm_filter): New.
(gpgsm_decrypt): Use this filter if requested.  Check authtag.

Note that the sample message pwri-sample.gcm.p7m is broken: The
authtag is duplicated to the authEncryptedContentInfo. I used a
temporary code during testing hack to that test message out.