send-email: signedoffcc -> signedoffbycc, but handle both

send-email: signedoffcc -> signedoffbycc, but handle both

The documentation now mentions sendemail.signedoffbycc instead
of sendemail.signedoffcc in order to match with the options
--signed-off-by-cc; the code has been updated to reflect this
as well, but sendemail.signedoffcc is still handled.

Signed-off-by: Michael Witten <mfwitten@mit.edu>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>