vt: more carefully handle vt_allocate_keyboard grab work

vt_allocate_keyboard only needs to unwind the effects of keyboard-grabbing,
rather than any associated vt window action that may have also happened.

Split out the bits that do the keyboard work into *_noswitch equivalents,
and use those in keyboard allocation. This will be less error-prone when a
later change will offer up different window state behavior when the console
is ungrabbed.

Reviewed by: ray
Differential Revision: https://reviews.freebsd.org/D27110