Return non-nil from gnutls-available-p under GnuTLS 2.x

* doc/misc/emacs-gnutls.texi (Help For Users): Update the
documentation of 'gnutls-available-p'.

* etc/NEWS (GnuTLS): Mention the change in the value returned by
'gnutls-available-p'.

* src/gnutls.c (Fgnutls_available_p): Return non-nil when GnuTLS
2.x is available.  (Bug#29147)

doc/misc/emacs-gnutls.texi

@@ -94,7 +94,11 @@ There's one way to find out if GnuTLS is available, by calling
 Zaretskii) in the same directory as Emacs, you should be OK.
 
 @defun gnutls-available-p
-This function returns @code{t} if GnuTLS is available in this instance of Emacs.
+This function returns non-@code{nil} if GnuTLS is available in this
+instance of Emacs, @code{nil} otherwise.  If GnuTLS is available, the
+value is a list of GnuTLS capabilities supported by the installed
+GnuTLS library, which depends on the library version.  The meaning of
+the capabilities is documented in the doc string of this function.
 @end defun
 
 Oh, but sometimes things go wrong.  Budgets aren't balanced,

etc/NEWS

@@ -1535,6 +1535,10 @@ See the node "(elisp) Checksum/Hash" in the ELisp manual for details.
 and 'gnutls-symmetric-decrypt'.
 See the node "(elisp) GnuTLS Cryptography" in the ELisp manual for details.
 
++++
+** The function 'gnutls-available-p' now returns a list of capabilities
+supported by the GnuTLS library used by Emacs.
+
 +++
 ** Emacs now supports records for user-defined types, via the new
 functions 'make-record', 'record', and 'recordp'.  Records are now

src/gnutls.c

@@ -2422,6 +2422,8 @@ GnuTLS AEAD ciphers     : the list will contain `AEAD-ciphers'.  */)
 
 #ifdef HAVE_GNUTLS
 
+  capabilities = Fcons (intern("gnutls"), capabilities);
+
 # ifdef HAVE_GNUTLS3
   capabilities = Fcons (intern("gnutls3"), capabilities);
   capabilities = Fcons (intern("digests"), capabilities);