Definitions shared between userland and kernel space moved to
include/rtdm/uapi/, so that userland may solely refer to include/rtdm,
which does not expose any kernel-specific bits anymore.

At this chance, we drop the useless prefixes/suffixes in interface
header names, since we can't have name conflicts when referring to
files the recommended way (i.e. <rtdm/file.h>):

rtdm/rtdm_driver.h -> rtdm/driver.h
rtdm/rtcan.h -> rtdm/can.h
rtdm/rtserial.h -> rtdm/serial.h
rtdm/rttesting.h -> rtdm/testing.h
rtdm/rtipc.h -> rtdm/ipc.h