Rather than using an hardcoded reclaim age, rely on an LRU-like approach
for dirhash cache, setting a target percent to reclaim (exposed via
SYSCTL). This allows to always make some amount of progress keeping the maximum
reclaim age dynamic.

Tested by:      pho
Reviewed by:	jhb
vt(4): The cursor coordinates are relative to the drawn area

... not the whole screen. Don't use font offsets in
vt_mark_mouse_position_as_dirty().

This fixes a bug where the mouse position wasn't marked as dirty when
approaching the borders of the drawn area.

MFC after:	1 week