url-mode: store URL in UTF-8, not UTF-32

The only time the URL is actually in UTF-32 is when we’re collecting
it (auto-detecting it) from the grid, since cells store their
character(s) in UTF-32.

Everything *after* that prefers the URL in UTF-8. So, do the
conversion while collecting the URL.

This patch also changes the URL activation code to strip the
‘file://user@host/’ prefix from file URIs that refer to files
on the *local* computer.

terminal.h

@@ -227,7 +227,7 @@ typedef tll(struct ptmx_buffer) ptmx_buffer_list_t;
 
 enum url_action { URL_ACTION_COPY, URL_ACTION_LAUNCH };
 struct url {
-    wchar_t *url;
+    char *url;
     wchar_t *text;
     wchar_t *key;
     struct coord start;