input: don't deliver mouse events if mouse area is not set

This caused the OSC to be always visible at startup on X11:
- EnterNotify event send a mouse event to input.c
- OSC has not completely initialized yet, and no mouse area is set
- mouse event is dispatched to "showhide" OSC section
- OSC becomes visible, regardless of mouse position

Fix this by treating the mouse area as empty if it's not set, instead of
infinite as it was before this commit. This means an input section must
set a mouse area to receive mouse events at all. We also have to change
the default section to receive mouse events with the new behavior.
lua: add new input flag from previous commit
input: don't print warning if certain internal keys are not bound

This affects MOUSE_MOVE and MOUSE_LEAVE. Both are needed internally
(such as for the OSC), but not really useful for input.conf. Since the
warning has the purpose of notifying the user that a key is unmapped and
what key name to use for setting up a binding in input.conf, the warning
is rather useless in this case. It's also annoying in combination with
the
--no-input-default-bindings option, since that removes the default
bindings to "ignore" for these keys.