gnutls_ciphersuite_get: new function to get unique ciphersuite name

gnutls_ciphersuite_get: new function to get unique ciphersuite name

The existing method to obtain the name of the currently negotiated TLS
ciphersuite is as follows:

- call gnutls_cipher_get, gnutls_mac_get, gnutls_kx_get
- call gnutls_cipher_suite_get_name with the value from the above functions

This process is cumbersome and only works with TLS 1.2 or earlier;
moreover the returned names are GnuTLS specific.

This change adds a new function gnutls_ciphersuite_get to eliminate
those limitations.  It returns the "canonical" name of the
ciphersuite, which is mostly identical to the ones registered in IANA,
with an exception for compatibility.

Signed-off-by: Daiki Ueno <ueno@gnu.org>