GnuTLS HMAC and symmetric cipher support

GnuTLS HMAC and symmetric cipher support

    * etc/NEWS: Add news for new feature.

    * doc/lispref/text.texi (GnuTLS Cryptography): Add
    documentation.

    * configure.ac: Add macros HAVE_GNUTLS3_DIGEST,
    HAVE_GNUTLS3_CIPHER, HAVE_GNUTLS3_AEAD, HAVE_GNUTLS3_HMAC.

    * src/fns.c (Fsecure_hash_algorithms): Add function to list
    supported `secure-hash' algorithms.
    (extract_data_from_object): Add data extraction function that
    can operate on buffers and strings.
    (secure_hash): Use it.
    (Fsecure_hash): Mention `secure-hash-algorithms'.

    * src/gnutls.h: Include gnutls/crypto.h.

    * src/gnutls.c (Fgnutls_ciphers, gnutls_symmetric_aead)
    (gnutls_symmetric, Fgnutls_symmetric_encrypt, Fgnutls_symmetric_decrypt)
    (Fgnutls_macs, Fgnutls_digests, Fgnutls_hash_mac, Fgnutls_hash_digest)
    (Fgnutls_available_p): Implement GnuTLS cryptographic integration.

    * test/lisp/net/gnutls-tests.el: Add tests.