contrib: better support symbolic port names in git-credential-netrc

contrib: better support symbolic port names in git-credential-netrc

To improve support for symbolic port names in netrc files, this
changes does the following:

 - Treat symbolic port names as ports, not protocols in git-credential-netrc
 - Validate the SMTP server port provided to send-email
 - Convert the above symbolic port names to their numerical values.

Before this change, it was not possible to have a SMTP server port set
to "smtps" in a netrc file (e.g. Emacs' ~/.authinfo.gpg), as it would
be registered as a protocol and break the match for a "smtp" protocol
host, as queried for by git-send-email.

Signed-off-by: Maxim Cournoyer <maxim@guixotic.coop>
Signed-off-by: Junio C Hamano <gitster@pobox.com>