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>

perl/Git.pm

@@ -1061,6 +1061,19 @@ sub _close_cat_blob {
 	delete @$self{@vars};
 }
 
+# Given PORT, a port number or service name, return its numerical
+# value else undef.
+sub port_num {
+    my ($port) = @_;
+
+    # Port can be either a positive integer within the 16-bit range...
+    if ($port =~ /^\d+$/ && $port > 0 && $port <= (2**16 - 1)) {
+        return $port;
+    }
+
+    # ... or a symbolic port (service name).
+    return scalar getservbyname($port, '');
+}
 
 =item credential_read( FILEHANDLE )