Use gnutls_rnd instead of gcry_randomize on newer GnuTLS versions.

This is a modified version of 690c372. Fixes #905.
Merge pull request #944 from SaberUK/insp20+gnutls-rnd

[2.0] Use gnutls_rnd instead of gcry_randomize on newer GnuTLS versions.
Remove some dead code found by Coverity