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.

MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271871 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f