s4:rpc_server: implement security context multiplexing
There're some systems like Cisco ISE use security multiplexing
without checking (via bind time feature negotiation)
the server supports it.
Others like VMWare View, fallback to NT4 style netlogon
connections without using netlogon secure channel,
which then triggers an error, with "server schannel = yes",
see https://bugzilla.samba.org/show_bug.cgi?id=13464.
BUG: https://bugzilla.samba.org/show_bug.cgi?id=7113
BUG: https://bugzilla.samba.org/show_bug.cgi?id=11892
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>