Use GnuTLS for SHA256 and HMAC SHA256
This is the first part of a huge patchset to get rid of our own crypto and move to GnuTLS.
This changes the requirement of GnuTLS to version 3.2 or newer for Samba! For Samba AD with MIT Kerberos we already require GnuTLS 3.4.7 (backupkey implementation). However to still work on older distribution I've chosen a version is known to work with the features we need and available on most LTS Linux distros (CentOS, RHEL, SLE and Ubuntu LTS). As GnuTLS 3.0 move to libnettle instead of gcrypt all the legacy gcrypt code has been removed!
We already have code which requires newer GnuTLS versions, however for such features configure checks are in place to only turn them on if the feature is available.
I've more or less tested each commit and often run version with old an new crypto in the server or smbtorture to make sure nothing breaks.
Please review carefully. Thanks!