MAINTAINERS, get_maintainer.pl: Add Designated Reviewer (R:) role

MAINTAINERS, get_maintainer.pl: Add Designated Reviewer (R:) role

The syntax has been copied from the Linux Maintainers file. I moved the following Linux
get_maintainer.pl patches to Xen, fixing up some merge issues (and a bug).

The get_maintainer.pl changes were based on the following git commits
* https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/scripts/get_maintainer.pl?id=
* c1c3f2c906e35bcb6e4cdf5b8e077660fead14fe
* 4f07510df2e8c47fd65b8ffaaf6c5d334d59d598

I also removed code related to
  P: Person (obsolete)
which is in the Linux MAINTAINER's file, but not ours. I may not have
caught all instances though.

I have tested on a number of files using mock entries in MAINTAINERS
using ./scripts/get_maintainer.pl -f ...

I also tested --nor to disable the support and it worked as expected.

Cc: Lars Kurth <lars.kurth@citrix.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: George Dunlap <George.Dunlap@eu.citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Julien Grall <julien.grall@arm.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Tim Deegan <tim@xen.org>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Juergen Gross <jgross@suse.com>
Signed-off-by: Lars Kurth <lars.kurth@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>