send-email: Cleanup smtp-domain and add config

send-email: Cleanup smtp-domain and add config

The way the code stored --smtp-domain was unlike its handling of other
similar options.  Bring it in line with the others by:

- Renaming $mail_domain to $smtp_domain to match the command line
  option. Also move its declaration from near the top of the file to
  near other option variables.

- Removing $mail_domain_default.  The variable was used once and only
  served to move the default away from where it gets used.

- Adding a sendemail.smtpdomain config option.  smtp-domain was the
  only SMTP configuration option that couldn't be set in the user's
  .gitconfig.

Signed-off-by: Brian Gernhardt <brian@gernhardtsoftware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>