vt(4): Save/restore keyboard mode & LED states when switching window

(MFC of r272416)

Add new functions to manipulate these mode & state, instead of calling
kbdd_ioctl() everyhere.

This fixes at least two bugs:

    1. The state of the Scroll Lock LED and the state of scroll mode
       could be out-of-sync. For instance, if one enables scroll mode on
       window #1 and switches to window #2, the LED would remain on, but
       the window wouldn't be in scroll mode.

       Similarily, when switching between a console and an X.Org
       session, the LED states could be inconsistent with the real
       state.

    2. When exiting from an X.Org session, the user could be unable to
       type anything. The workaround was to switch to another console
       window and come back.

Differential Revision:	https://reviews.freebsd.org/D821
Reviewed by:	ray@
Approved by:	ray@
Tested by:	kwm@