services: xorg: Define an <xorg-configuration> record type.

* gnu/services/xorg.scm (<xorg-configuration>): New record type.
(xorg-configuration-file): Remove.
(xorg-wrapper): Remove #:modules, #:configuration-file, and
 #:xorg-server; add optional 'config' parameter instead.  Adjust
accordingly.
(xorg-start-command): Likewise.
* doc/guix.texi (X Window): Document 'xorg-configuration'.  Update
'xorg-start-command' documentation.  Remove 'xorg-configuration-file'
documentation.