Update to v3.6.8

Changes
=======

* Version 3.6.8 (released 2019-05-28)

** libgnutls: Added gnutls_prf_early() function to retrieve early keying
  material (#329)

** libgnutls: Added support for AES-XTS cipher (#354)

** libgnutls: Fix calculation of Streebog digests (incorrect carry operation in
  512 bit addition)

** libgnutls: During Diffie-Hellman operations in TLS, verify that the peer's
  public key is on the right subgroup (y^q=1 mod p), when q is available (under
  TLS 1.3 and under earlier versions when RFC7919 parameters are used).

** libgnutls: the gnutls_srp_set_server_credentials_function can now be used
  with the 8192 parameters as well (#995).

** libgnutls: Fixed bug preventing the use of gnutls_pubkey_verify_data2() and
  gnutls_pubkey_verify_hash2() with the GNUTLS_VERIFY_DISABLE_CA_SIGN flag (#754)

** libgnutls: The priority string option %ALLOW_SMALL_RECORDS was added to allow
  clients to communicate with the server advertising smaller limits than 512

** libgnutls: Apply STD3 ASCII rules in gnutls_idna_map() to prevent
  hostname/domain crafting via IDNA conversion (#720)

** certtool: allow the digital signature key usage flag in CA certificates.
  Previously certtool would ignore this flag for CA certificates even if
  specified (#767)

** gnutls-cli/serv: added the --keymatexport and --keymatexportsize options.
  These allow testing the RFC5705 using these tools.

** API and ABI modifications:
gnutls_prf_early: Added
gnutls_record_set_max_recv_size: Added
gnutls_dh_params_import_raw3: Added
gnutls_ffdhe_2048_group_q: Added
gnutls_ffdhe_3072_group_q: Added
gnutls_ffdhe_4096_group_q: Added
gnutls_ffdhe_6144_group_q: Added
gnutls_ffdhe_8192_group_q: Added

(sevan)