Various auth_rewrite/splay_userauth fixes by Robert Collins

Various auth_rewrite/splay_userauth fixes by Robert Collins

* auth_modules is no longer a top-level subdir. Don't try to reference
it from the makefile.

* more name changes from proxyauth to user, missed by Henrik

* corrected addition to a existing user list

* fixed a coredump on shutdown with empty user lists (i.e. only REQUIRED)

* fixed a reconfigure bug where the authentication shemes got somewhat trashed

* added a debug log message when a new scheme is added