git-send-email.perl - try to give real name of the calling host to HELO/EHLO

git-send-email.perl - try to give real name of the calling host to HELO/EHLO

Add new functions maildomain_net(), maildomain_mta() and
maildomain(), which return FQDN where possible for use in
send_message(). The value is passed to Net::SMTP HELO/EHLO
handshake. The domain name can also be set via new --smtp-domain
option.

The default value in Net::SMTP may not get through:

  Net::SMTP=GLOB(0x267ec28)>>> EHLO localhost.localdomain
  Net::SMTP=GLOB(0x267ec28)<<< 550 EHLO argument does not match calling host

whereas using the FQDN that matches the IP, the result is:

  Net::SMTP=GLOB(0x15b8e80)>>> EHLO host.example.com
  Net::SMTP=GLOB(0x15b8e80)<<< 250-host.example.com Hello host.example.com [192.168.1.7]

The maildomain*() code is based on ideas in Perl library
Test::Reporter by Graham Barr <gbarr@pobox.com> and Mark Overmeer
<mailtools@overmeer.net> released under the same terms as Perl
itself.

Signed-off-by: Jari Aalto <jari.aalto@cante.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>