isa_dmamap_create() calls to their open/close entrypoints. This worked
with some luck, but broke on i386 when _bus_dmamap_create started
to allocate bounce buffers upfront, since memory below 16M may well
not be available when the sound devices is opened for the Nth time.
To fix this, create a new simple interface, isa_drq_alloc/isa_drq_free,
wrappers around already existing bitmask macros. These are expected
to be used before an isa_dmamap_create call, and after an
isa_dmamap_destroy call, respectively. For the sb and ad1848 drivers,
they're deferred until open/close.
All isa_dmamap_create calls can now use BUS_DMA_ALLOCNOW and be done
at attach time.
