vt(4): CONS_HISTORY/CONS_CLRHIST should operate on issuing terminal

Currently the CONS_HISTORY and CONS_CLRHIST ioctls modify the state of the
active terminal instead of the terminal against which the ioctl was issued.
Because of the way vidcontrol(1) works, these are the same in most cases.
But a poorly-timed window switch can make them differ. This is reproducible
by issuing e.g. 'vidcontrol -s 2 && vidcontrol -C' to switch from vty 1 to
vty 2; teken will reset the cursor position on vty 1 but vt(4) will clear
the history buffer of vty 2, producing an interesting state of affairs.

Differential Revision: https://reviews.freebsd.org/D25564