xfree86/os: Add xf86MatchDMI stubs

Add stubs for DMI identifier initialization, cleanup, set and retrieval:
    xf86MatchDMI{Init,Cleanup,Set,Get}

Signed-off-by: Daniel Martin <consume.noise@gmail.com>
xfree86/linux: Implement xf86MatchDMIInit

Read DMI identifiers from sysfs (/sys/devices/virtual/dmi/id).

Signed-off-by: Daniel Martin <consume.noise@gmail.com>
xfree86: Add MatchDMI support in InputClass sections

This adds support for MatchDMI entries in InputClass sections.

It can be used to match on DMI identifiers applied to the maschine. This
might be usefull when it's not sufficient or possible to match on device
tags or something similar.

Signed-off-by: Daniel Martin <consume.noise@gmail.com>