send-email --smtp-server-port: allow overriding the default port

send-email --smtp-server-port: allow overriding the default port

You can use --smtp-server-port option to specify a port
different from the default (typically, SMTP servers listen
to smtp port 25 and ssmtp port 465).

Users should be aware that sending auth info over non-ssl
connections may be unsafe or just may not work at all
depending on SMTP server config.

Signed-off-by: Glenn Rempe <glenn@rempe.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>