Add MAINTAINERS file

The Xserver tree has several different maintainers for various pieces of the tree.

Historically these have been recorded in an entirely different tree (xorg-docs repo), which rips apart the information base and makes it pretty uncomfortable to find the maintainers (or other contacts) for some particular area of the Xserver. And it gets even more complex when the roles differing between branches (eg. stable 21.* line vs mainline vs. 3rdparty branches)

Optimally this should be possible to do automatically by something like the Linux kernel's get_maintainer.pl script, which also can be called by other tooling. (since the existing MAINTAINERS file in xorg-docs already has the Linux kernel's format, it's get_maintainer.pl script can be used here).

In general it's good if code bases are self-documenting, w/o having to consult additional sources.

This queue introduces MAINTAINERS file for the Xserver itself (once it's landed, the one in xorg-docs should be changed to just pointing here) as well as the Linux kernel's get_maintainer.pl script.

It's done in several stages:

1. copy over MAINTAINERS from xorg-docs (original commit ID recorded in commit message)
2. drop all entries for things outside the Xserver tree
3. add some yet missing entries (Jeremy and myself) as well as some missing lines
4. copy over get_maintainer.pl from Linux tree (also recording original commit ID)
5. tweak it to work with the Xserver tree (source tree detection)

Once this this has landed mainline, I'll post a MR on xorg-devel for replacing the Xserver entries there by a link to this new MAINTAINERS file.