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vt(4): Fix out-of-bounds array access in VT_ACTIVATE ioctl handling

CID:		1229964

vt(4): Use strncpy() to copy into a fixed-size buffer

CID:		1230007

vt(4): Remove vt_buf->vb_dirtymask

This structure and the associated functions were unused since the
implementation of vd_bitblt_text_t callbacks.

vt(4): Rewrite history scrolling

It's now possible to scroll up the 500 hard-coded lines of history, not
just a fraction of them. For instance, one can reach the top of the boot
process.

Sometimes, when scrolling or when changing the screen size (by changing
the font or loading a KMS driver for instance), one could see the
history cycling (old content appeared below latest lines). This is
fixed.

Now, when the resolution changes are more lines can be shown, the
displayed area is adjusted so that, if the screen was filled with
content before, it's filled with content after as well: more history
is visible, instead of having blank lines below the previously visible
content.

vt(4): Remove superfluous word in comment

Submitted by:	brueffer@

Make gcc happy by initialising the variable only set in a couple of
case statements without a default.