send-email: get all the quoting of realnames right

send-email: get all the quoting of realnames right

- when sending several mails I got a slightly different behaviour for the first
  mail compared to the second to last one.  The reason is that $from was
  assigned in line 608 and was not reset when beginning to handle the next
  mail.

- Email::Valid can only handle properly quoted real names, so quote arguments
  to extract_valid_address.

This patch cleans up variable naming to better differentiate between sender of
the mail and it's author.

Signed-off-by: Uwe Kleine-König <ukleinek@informatik.uni-freiburg.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>