prevent users from registering with name that is not a valid wikifile

avoids XSS attacks and is generally a good limitation

IkiWiki/CGI.pm

@@ -118,6 +118,7 @@ sub cgi_signin ($$) { #{{{
 				validate => sub {
 					my $name=shift;
 					length $name &&
+					$name=~/$wiki_file_regexp/ &&
 					! userinfo_get($name, "regdate");
 				},
 			);