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

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@
MFC after: 3 days