get_maintainer: Teach get_maintainer.pl about the new "R:" tag

get_maintainer: Teach get_maintainer.pl about the new "R:" tag

We can now designate reviewers in the MAINTAINERS file with the new
"R:" tag, so this commit teaches get_maintainers.pl to add their
email addresses.

Cherry picked from Linux commit c1c3f2c906e35bcb6e4cdf5b8e077660fead14fe,
with fixes to avoid \C as in QEMU commit ba10f729f1 ("get_maintainer.pl:
\C is deprecated", 2015-09-25).

Signed-off-by: Joe Perches <joe@perches.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>