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

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.

PR: 224436
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D25079