from os docs, rework posix docs.

Georg Brandl has added fchmod() and fchown(). I've contributed lchown but I'm not able to test it on Linux. However it should be available on Mac and some other flavors of Unix.
I've made a quick test of fchmod() and fchown() on my system. They are working as expected.

Also document a few other O_ constants that were missing from documentation.

confuse.

Added a note about Python 3's collections.Mapping etc., above section
that describes isMappingType() etc.

Added xrefs between os, os.path, fileinput, and open().