urls: OSC-8 URLs can now optionally be underlined outside of url-mode

This patch adds a new configuration option,
‘osc8-underline=url-mode|always’.

When set to ‘url-mode’, OSC-8 URLs are only
highlighted (i.e. underlined) in url-mode, just like auto-detected
URLs.

When set to ‘always’, they are always underlined, regardless of mode,
and regardless of their other attributes.

This is implemented by tagging collected URLs with a boolean,
instructing urls_render() and urls_reset() whether they should update
the cells’ ‘url’ attribute or not.

The OSC-8 collecter sets this based on the value of ‘osc8-underline’.

Finally, when closing an OSC-8 URL, the cells are immediately tagged
with the ‘url’ attribute if ‘osc8-underline’ is set to ‘always’.

terminal.h

@@ -256,6 +256,7 @@ struct url {
     struct coord start;
     struct coord end;
     enum url_action action;
+    bool url_mode_dont_change_url_attr; /* Entering/exiting URL mode doesn’t touch the cells’ attr.url */
 };
 typedef tll(struct url) url_list_t;