mail/sendmail: Add port option for SMTPUTF8 support, enabled by default.

Postfix has had support for quite some time, compiled in by default
http://www.postfix.org/SMTPUTF8_README.html

Exim has support but it's not a default build option
http://www.exim.org/exim-html-current/doc/html/spec_html/ch-internationalisation.html

Sendmail had no support until Arnt produced this patch.

As far as commercial access to SMTPUTF8 email goes:

GMail has support for SMTPUTF8 for 3 years
Outlook.com will have support shortly

The world is moving forward with SMPTUTF8 and if it does not get wider
distribution it will restrict the ability to self-host email as the only
solution to send and receive email from these users will be to use a
commercial cloud-hosted email platform.

This patch is a no-op unless you are sending mail that requires the
SMTPUTF8 extension. If you are running a Sendmail server and you did not
have this patch you would have received errors and the messages would
have been undeliverable.

More info: http://rant.gulbrandsen.priv.no/eai/one-minute-guide

PR: 224452
Reviewed by: Gregory Shapiro, Claus Assmann (private mails from Arnt)
Obtained from: Arnt Gulbrandsen
Sponsored by: CNNIC