vt(4): Remove vd_bitbltchr_t

It's replaced by vd_bitblt_text_t, which gives more context to the
backend and allows it to perform more efficiently when redrawing a given
area.

MFC after:	1 week
Use refcount_init in sigacts_alloc.

This change is a no-op, but fixes up an inconsistency introduced with
r268634.

MFC after:	3 days