bindings: let ^L put the cursor line at center, then top, then bottom

Something similar can be achieved with:

  bind ^T "{center}{bottomrow}{center}{toprow}" main
  bind ^B "{center}{toprow}{center}{bottomrow}" main

But that requires allocating two extra shortcuts, and it works right only
when the edit window has an odd number of rows, not with an even number.
Also, this new ^L behavior is available by default, out-of-the-box.

src/global.c

@@ -1434,7 +1434,7 @@ void shortcut_init(void)
 	add_to_sclist(MEXECUTE, "^J", 0, do_full_justify, 0);
 #endif
 #ifndef NANO_TINY
-	add_to_sclist(MMAIN, "^L", 0, do_center, 0);
+	add_to_sclist(MMAIN, "^L", 0, do_cycle, 0);
 #endif
 	add_to_sclist(MMOST|MBROWSER|MHELP|MYESNO, "^L", 0, full_refresh, 0);