Support Generic GSS-API Mechanisms

Previously, OpenSSH required that mechanisms be registered in a static
table within the ssh server.  There are three reasons for this:

* To meet requirements from RFC 4462 about which mechanisms are used

* To have mechanism-specific code to determine whether a user is authorized

* A mechanism-specific implementation of localname

Since then the GSS-API vendors have added support for gss_localname
and gss_userok.  In addition, mechanism attributes allow sshd to probe
for which mechanisms meet the requirements from RFC 4462.

Add support for arbitrary GSS-API mechanisms using gss_localname and
gss_userok.  Filter out negotiation mechanisms.