uterm: vt: handle VT switches without kernel input

uterm: vt: handle VT switches without kernel input

We now set KBMODE to K_OFF so we are totally independent of kernel input.
Instead, we handle VT switches with uterm-input now. This also allows us
to have full control of which keyboard input is parsed by us and which is
parsed by the kernel.

We still need to set a flag for uterm-input events that they were handled
to avoid having the TSM layer handle these events again.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>