Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

Rename busdma_sync() to busdma_sync_range() and rename the
base and size parameters to ofs and len (resp). Add a new
busdma_sync() that makes the entire MD coherent.

Implement busdma_md_unload() and busdma_sync().

While here:
1. have the Python bindings contain constants for the space
identifiers and the sync operation.
2. change the segment iterators to return None when done,
not ENXIO.

Add busdma_md_create, busdma_md_destroy and busdma_md_load.

Add accessor functions for iterating over segments. A segment
can be in bus address space, physical memory space or virtual
memory space.

Add busdma_mem_alloc & busdma_mem_free.

Add DMA tag management to the C library and Python binding.