Add bsearch_b to the libc map and the stdlib header.

bsearch_b is the Apple blocks enabled version of bsearch(3).
This was added to libc in Revision 264042 but the commit
missed the declaration required to make use of it.

While here move some other block-related functions to the
BSD_VISIBLE block as these are non-standard.

Phabric:	D638
Reviewed by:	theraven, wollman
Create a mechanism for looking up a device_t associated with an ofw/fdt
xref handle, and for registering that association.  Also use the same data
for faster translations between node and xref handles.

Now when fdt properties contain &othernode references, a driver can find
the device instance that corresponds to &othernode, and thus can use
interfaces provided by that instance.

Reviewed by:	nwhitehorn