Revert to using yp_order() to probe for master.paswd.by* maps and

Revert to using yp_order() to probe for master.paswd.by* maps and
don't probe the server at all for passwd.by* maps. This fixes
interoperability with the Services For UNIX NIS server (which is
really a front end to Captive^WActiveDirectory). This server
incorrectly returns success for all YPPROC_MASTER requests,
even for maps that don't exist, which makes it impossible to
(ab)use it to probe for the existence of the master.passwd.by*
maps.

This is a little kludgey, but basically restores the original
behavior of getpwent.c as it is in -stable, and works around both
the lack of YPPROC_ORDER on NIS+ servers as well as the broken
YPPROC_MASTER on Services For UNIX servers.