r11521: Add in client support for checking supportedSASLmechanisms, and then

r11521: Add in client support for checking supportedSASLmechanisms, and then
determining a mechanism to use.

Currently it doesn't to fallbacks like SPNEGO does, but this could be
added (to GENSEC, not to here).

This also adds a new function to GENSEC, which returns a list of SASL
names in our preference order (currently determined by the build
system of all things...).

Also make the similar function used for OIDs in SPNEGO do the same.

This is all a very long-winded way of moving from a hard-coded NTLM to
GSS-SPNEGO in our SASL client...

Andrew Bartlett
(This used to be commit 130eb9bb9a37957614c87e0e6846a812abb51e00)