Add scrollback support to the openurlonclick patch

The openurlonclick and scrollback patches are now working together,
so links can be clicked in the scrollback buffer too. This update also
adds url underlining and other improvements to the openurlonclick patch.

The full list of changes in the openurlonclick patch:
- Adds scrollback support
- Adds modkey option
- Better url detection
- Underlines url when the mouse pointer is over a link
- Opens a browser as a background process, so it won't lock the terminal anymore
- Fixes a segmentation fault bug

st.h

@@ -219,13 +219,16 @@ typedef struct {
 	#endif // BACKGROUND_IMAGE_PATCH
 	Visual *vis;
 	XSetWindowAttributes attrs;
-	#if HIDECURSOR_PATCH
+	#if HIDECURSOR_PATCH || OPENURLONCLICK_PATCH
 	/* Here, we use the term *pointer* to differentiate the cursor
 	 * one sees when hovering the mouse over the terminal from, e.g.,
 	 * a green rectangle where text would be entered. */
 	Cursor vpointer, bpointer; /* visible and hidden pointers */
 	int pointerisvisible;
 	#endif // HIDECURSOR_PATCH
+	#if OPENURLONCLICK_PATCH
+	Cursor upointer;
+	#endif // OPENURLONCLICK_PATCH
 	int scr;
 	int isfixed; /* is fixed geometry? */
 	#if ALPHA_PATCH