git-send-email fixes, cleanups and improvements

The following series of patches perform a number of fixes, cleanups and
improvements to git-send-email.

These were spurred by the dual factors of git-send-email losing some of my
email due to the envelope sender, as well as the malformatting of the headers
(due to the period after my middle initial) that caused strange breakages with
some sendmail binaries.

01/09  Document --dry-run parameter to send-email.
02/09  Prefix Dry- to the message status to denote dry-runs.
03/09  Debugging cleanup improvements
04/09  Change the scope of the $cc variable as it is not needed outside of send_message.
05/09  Perform correct quoting of recipient names.
06/09  Validate @recipients before using it for sendmail and Net::SMTP.
07/09  Ensure clean addresses are always used with Net::SMTP
08/09  Allow users to optionally specify their envelope sender.
09/09  Document --dry-run and envelope-sender for git-send-email.