Make sysmouse(4) use its own locks, instead of using Giant.

Make sysmouse(4) use its own locks, instead of using Giant.

When I changed syscons(4) to work with the MPSAFE TTY code, I just
locked all device nodes down using the compatibility feature that allows
you to override the TTY's lock (Giant in this case). Upon closer
inspection, it seems sysmouse(4) only has two internal variables that
need locking: mouse_level and mouse_status.

I haven't done any performance benchmarks on this, though I think it
won't have any dramatic improvements on the system. It is good to get
rid of Giant here, because the third argument of tty_alloc() has only
been added to ease migration to MPSAFE TTY. It should not be used when
not needed.

While there, remove SC_MOUSE, which is a leftover from the MPSAFE TTY
import.