Fix version numbers for some GnuTLS features

Fix version numbers for some GnuTLS features

Problem reported by Glenn Morris (Bug#27708#58).
* src/gnutls.c (HAVE_GNUTLS_X509_SYSTEM_TRUST):
New macro.  Use it instead of low-level version number checks.
(HAVE_GNUTLS_AEAD): Move here from gnutls.h, and rename from
HAVE_GNUTLS3_AEAD.  All uses changed.  Indent preprocessor lines.
* src/gnutls.h (HAVE_GNUTLS3_CIPHER, HAVE_GNUTLS3_DIGEST)
(HAVE_GNUTLS3_HMAC): Remove, since these were available
before GnuTLS 3.0.0 and the code checks them only if HAVE_GNUTLS3
is defined.  Remove all uses; this simplifies the code a bit.