Push sam_get_server_info_principal into the auth subsystem

Push sam_get_server_info_principal into the auth subsystem

This means it must be accessed via the supplied auth_context in the
GENSEC server, and should remove the hard depenceny of GENSEC on the
auth subsystem and ldb (allowing LDB not to rely on LDB is considered
a good thing, apparently)

Andrew Bartlett