Rewrite Mac OS X support to use proper XNU headers

Move from one set of headers per major OS X release to one set of
headers per XNU release.  Fetch the header files from Apple's official
Open Source site instead of GitHub in the fetchdeps developer target.
As a side effect, 10.6.x is now supported as well (untested), and proper
headers are used for 10.10.

Issue:		#39