uspace: Allow debugging rtapi_app as root

e.g., with
    sudo env RTAPI_UID=`id -u` valgrind bin/rtapi_app
or
    sudo env RTAPI_UID=`id -u` gdb bin/rtapi_app
rtapi_app will be executed by root but use your normal user's UID to
create shared memory segments, etc.  Once rtapi_app is running, you
can communicate with it using `halcmd` as usual (not halrun; this would
report that realtime was already running).