Currently there is a single file, xorg.conf, for configuring the server.
This works fine most of the time, but it becomes a problem when packages
or system services need to adjust the configuration. Instead, allow
multiple configuration files to live in a directory. Typically this will
be /etc/X11/xorg.conf.d.

Files with a suffix of .conf will be read and added to the server
configuration after xorg.conf. The server won't fall back to using the
auto configuration unless there is no config file and there are no files
in the config directory.

Right now this uses a simpler search template than the config file
search path by not using the command line or environment variable
parameters. The matching code was refactored a bit to make this more
coherent. Any DDX wanting to read the config files will need to call
xf86initConfigFiles before opening/reading them. This is to allow
xf86openConfigFile without xf86openConfigDirFiles and vice-versa.

Signed-off-by: Dan Nicholson <dbn.lists@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer at who-t.net>