Remove unused EMC2_CONFIG_DIR make variable and cpp #define
rtapi/Submakefile:  remove fixed FIXME re: RTAI -llxrt in LDFLAGS
Update realtime and rtapi.conf scripts to handle shmdrv

If USE_SHMDRV=yes in rtapi.conf, add it to the MODULES list and
load/unload it.  If USE_SHMDRV=yes, it is an error if the module can't
be loaded.

Several pieces of code unified so shmdrv loads as a module in
userspace just as other kthreads modules:

- Find shmdrv.ko module in rtlib/shmdrv/kver for userland threads
- Extend kernel checks for userland when USE_SHMDRV=yes
- MODPATH may contain multiple base paths to handle RTAI's separate module dir
  - This generalizes module location, avoiding yet another exception
  - (Don't bother exporting MODPATH; no consumers outside this script)
- FIXME:  Hack around userland threads mod_ext being .so
  - For userland, 'flavor -m' returns .so
  - But the shmdrv module will always be .ko, even for user threads
  - So, hardcode .ko for now until this is sorted out