Use GSS-SPNEGO if available

Currently adcli uses the GSSAPI SASL mechanism for LDAP authentication
and to establish encryption. While this works in general it does not
handle some of the more advanced features which can be required by AD
DCs.

The GSS-SPNEGO mechanism can handle them and is used with this patch by
adcli if the AD DC indicates that it supports it.

Related to https://bugzilla.redhat.com/show_bug.cgi?id=1762420