Add API to support AEAD cipher modes

* cipher/cipher.c (_gcry_cipher_authenticate, _gcry_cipher_checktag)
(_gcry_cipher_gettag): New.
* doc/gcrypt.texi: Add documentation for new API functions.
* src/visibility.c (gcry_cipher_authenticate, gcry_cipher_checktag)
(gcry_cipher_gettag): New.
* src/gcrypt.h.in, src/visibility.h: add declarations of these
functions.
* src/libgcrypt.defs, src/libgcrypt.vers: export functions.

Authenticated Encryption with Associated Data (AEAD) cipher modes
provide authentication tag that can be used to authenticate message. At
the same time it allows one to specify additional (unencrypted data)
that will be authenticated together with the message. This class of
cipher modes requires additional API present in this commit.

This patch is based on original patch by Dmitry Eremin-Solenikov.

Changes in v2:

• Change gcry_cipher_tag to gcry_cipher_checktag and gcry_cipher_gettag for giving tag (checktag) for decryption and reading tag (gettag) after encryption.
• Change gcry_cipher_authenticate to gcry_cipher_setaad, since additional parameters needed for some AEAD modes (in this case CCM, which needs the length of encrypted data and tag for MAC initialization).
• Add some documentation.

Changes in v3:

• Change gcry_cipher_setaad back to gcry_cipher_authenticate. Additional parameters (encrypt_len, tag_len, aad_len) for CCM will be given through GCRY_CTL_SET_CCM_LENGTHS.

Changes in v4:

• log_fatal => log_error

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>