send-email: validate patches before sending anything

send-email: validate patches before sending anything

We try to catch errors early so that we don't end up sending
half of a broken patch series. Right now the only validation
is checking that line-lengths are under the SMTP-mandated
limit of 998.

The validation parsing is very crude (it just checks each
line length without understanding the mailbox format) but
should work fine for this simple check.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>