vt(4): reset scrollback and cursor position after clearing history buffer

[jah]

Summary

r361601 implemented basic support for cleaing the console history buffer.
But after clearing the history buffer, it's not especially useful to be
able to scroll back through that buffer, or for the cursor position to
remain at (very likely) the bottom of the screen.

[jah]

sys/kern/subr_terminal.c

494

This exposes a new issue: before, it was practically guaranteed that the cursor would be at the bottom of the screen, so the CLRHIST ioctl would be immediately followed by a scroll event that would invalidate the entire visible rect. Now that may no longer be the case: for example if you issue 'vidcontrol -C', then do a few things that don't fill the screen, then issue 'vidcontrol -C' again, some prior artifacts might still remain visible. Let me see if I can fix that...

[jah]

Ensure the visible rect is invalidated on CONS_CLRHIST

[emaste]

sys/kern/subr_terminal.c

485

IMO it'd be fine to do if (cmd == CONS_CLRHIST) which neatly avoids the slightly confusing double } at the same indentation; we can deal with the awkwardness if we ever have a 2nd case to deal with. Yes, this is slightly inconsistent with the switch above.

[jah]

Light cleanup of cursor position handling