patman: add option for limiting the Cc list

Many mailing-lists consider a long Cc list a sign of spam and will
either drop the message or mark it for moderation. Because patman
automatically invokes get_maintainer.pl the Cc list can expand
unexpectedly. Allow the user to specify a limit for the Cc list.

This limit is applied after removing any known bouncing addresses. By
default no limit is applied.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>