vt(4): Adjust the cursor position after changing the window size

A new terminal_set_cursor() is added: it wraps the existing
teken_set_cursor() function.

In vtbuf_grow(), the cursor position is adjusted at the end of the
function. In vt_change_font(), we call terminal_set_cursor() just after
terminal_set_winsize_blank(), while the terminal is mute.

This fixes a bug where, after loading a kernel video driver which
increases the terminal window size, the cursor remains at its old
position, in other words, in the middle of the display content.

PR:		194421
MFC of:		r273932
vt(4): Fix keyboard allocation when kbdmux(4) isn't used

The problem was that only the kbdmux keyboard index was saved in
vd->vd_keyboard. This index is -1 when kbdmux isn't used. In this
case, the keyboard was correctly allocated, but the returned index was
discarded.

PR:		194718
MFC of:		r273973
drm: Lower priority of three messages related to invalid EDID

Like in r259717, the prority goes from "error" to "debug" to avoid
spamming logs when the connectors are polled.

PR:		194770
Submitted by:	Larry Rosenman <ler@lerctr.org>
MFC of:		r273962, r274587