Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

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.

Change the dev argument from a full path to just the device
identification (e.g. isa:0x3f0 or pci0:2:1:0). In libbus,
the device is turned into a path name. For bus_space_map(),
the resource is now specified in a second argument.
Before:
bus.map('/dev/proto/pci0:2:1:0/pcicfg')
busdma.tag_create('/dev/proto/pci0:2:1:0/busdma', ...)
Now:
bus.map('pci0:2:1:0', 'pcicfg')
busdma.tag_create('pci0:2:1:0', ...)

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.

Rename bus_space to bus (i.e. drop _space). This makes the
API match the convenience macros in <sys/bus.h>. Bus space
can now reference both bus and busdma.

Add busdma_mem_alloc & busdma_mem_free.

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

Add a C libary and a Python module that implements an API similar
to bus_space(9) and that uses the proto(4) driver for talking to
hardware. If the I/O resource is a memory mapped I/O resource,
then mmap(2) will be attempted to avoid read(2)/write(2) overhead.

Sponsored by: Juniper Networks, Inc.