url-mode: remove overlapping URLs, not just duplicates

echo -e '\e]8;;https://www.foo.bar\e\\https://www.foo\e]8;;\e\\.bar'

will produce an OSC-8 URL (https://www.foo) that is slightly shorter
than the auto-detected one (https://www.foo.bar).

This produces strange results in URL mode. For example, if
url.osc8-underline=always, the OSC8 underline will be removed when
url-mode is exited.

This patch changes the behavior so that auto-detected URLs that
overlap OSC-8 URLs are removed.

Note that OSC-8 URLs cannot overlap with each other, and that
auto-detected URLs also cannot overlap with each other.

terminal.h

@@ -258,6 +258,7 @@ struct url {
     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 */
+    bool osc8;
 };
 typedef tll(struct url) url_list_t;