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