rtapi-uspace: Ask gcc for the semantics we want

I know for that hm2 uses casts in a way forbidden by gcc without
-fno-strict-aliasing.  I suspect that there are places that depend on
wrapping semantics for signed arithmetic, such as in encoder wraparound,
which is what -fwrapv asks for.
rtapi: provide rtapi_io.h

.. which provides the full {in,out}[bwl] APIs, with and without
rtapi_ prefix.
rtapi: provide (non-functional) request/release region for uspace

.. in that they always succeed, but there's no effective prevention of
concurrent access to the same I/O ports by the kernel as there is in
kernel space.
rtapi: import EXPORT_SYMBOL_GPL for uspace

.. for now, this does not enforce the GPL linking
semantic, but this does not mean that violating
the GPL linking semantic is OK
serport: use <rtapi_io.h>

.. now this component is buildable for uspace with comp --install
serport: build for uspace
rtapi: provide additional portability headers

.. this set is sufficient to allow modified hostmot2
and hm2_test components to build.
rtapi-uspace: implement rtapi_delay
hostmot2: build hostmot2 and hm2_test for uspace
hm2-idrom: Let test succeed in uspace
rtapi: document new headers
build: we'll require libudev from now on
uspace: implementation of PCI APIs

.. and other APIs sufficient to build and run
hm2_pci hardware (tested with 5i20)

This work is based heavily on the PCI implementation in the "ubc3" branch,
a part I think was mostly based on Michael Buesch's work.
hm2_pci: rtapi-prefix APIs and enable for uspace
rtapi: document new header