rtapi/userthreads/pci: assure PCI support properly linked

rtapi_pci.o got lost along the way
rtapi.h: make rtapi_set_msg_handler() visible to both ULAPI and RTAPI

the message handler can be set in either environment and is applicable
only in that environment

This gets rid of the need to compile rtapi_app.cc with -DRTAPI.
rtapi_app: dont link with -ludev and -DRTAPI

technically rtapi_app isnt RTAPI or ULAPI; just the RT modules loaded
by rtapi_app are 'RTAPI' (meaning to have the RTAPI symbol space not to
shared with rtapi_app proper).

for convenience it's fine to use -DULAPI.