create new, extensible interface for (additional) auth backends
attempt to handle HANDLER_WAIT_FOR_EVENT returned by auth backends
to allow for async auth backends (e.g. to mysql database)
separate auth backends from mod_auth and http_auth
mod_authn_file.c htdigest, htpasswd, plain auth backends
mod_authn_ldap.c ldap auth backend
add http_auth.c to common_sources for auth backend registration
(mod_authn_file could be three separate modules, but no need for now)
