cgiurl(): return a locally-valid path by default

To get an absolute version you can use cgiurl(cgiurl => $config{cgiurl}).
The only place in IkiWiki that seems to actually need an absolute URL
is the openid plugin, and that already uses the named parameter.

IkiWiki.pm

@@ -1039,7 +1039,8 @@ sub linkpage ($) {
 sub cgiurl (@) {
 	my %params=@_;
 
-	my $cgiurl=$config{cgiurl};
+	my $cgiurl=$local_cgiurl;
+
 	if (exists $params{cgiurl}) {
 		$cgiurl=$params{cgiurl};
 		delete $params{cgiurl};