Fix gnutls_x509_trust_list_add_trust_mem with DER-format certificates.
The function took a "type" argument and then happily proceeded to ignore
it and try PEM format anyway.
Most importantly, this makes gnutls_x509_trust_list_add_system_trust
work on Windows, which loads DER certificates using this function. I'll
be damned if that actually ever worked properly -- certainly not in any
git version. :)
Also added test for gnutls_x509_trust_list_add_trust_mem.
Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>