patman: separate emails in CC list with NULs

There is a contributor in Linux kernel with a comma in their name, which
confuses patman and results in invalid to- or cc- addresses on some
patches. To avoid this, let's use \0 as a separator when generating cc
file.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>