Remove inclusion of headers exported by libosa.

As ohNet doesn't actually rely on the libosa headers anymore, we can reduce the
configuration to whatever is contained in CROSS_COMPILE, CROSS_ENDIAN, and CROSS_CPU.
Add File abstraction, provide ANSII C implementation for Windows, Posix.

Stub out for Volkano2
Fix Windows implementation of File.cpp (straight copy from Posix impl.)
Comment unreference informal parametes in Os/Windows/File.cpp

Another attempt to resolve Os/Windows/File.cpp build problem.
Fix for x64 build
Provide implementation of niceness setting, when ATTEMPT_THREAD_NICENESS is defined.

Off by default, obviously.

According to Posix, niceness is a per-process property, not per-thread. However,
pthreads in Linux can, and does, apply nicess as a per-thread property.

We try and exploit that here by mapping our priority range onto a range of
nice-levels that do not require privaledges. In practise this means that we can
only really demote threads, not promote them.
Fix argument ordering in ANSII C implementations of OpenHome::File.
Merge branch 'master' of ssh://core.linn.co.uk/home/git/ohnet

Conflicts:
	Os/Windows/File.cpp