patman: Use Patch-cc: instead of Cc:

Add a new Patch-cc: tag which performs the service now provided by
the Cc: tag. The Cc: tag is interpreted by git send-email but
ignored by patman.

So now:

  Cc: patman does nothing. (git send-email can cc patches)
  Patch-cc: patman Cc's patch and removes this tag from the patch

Signed-off-by: Simon Glass <sjg@chromium.org>