Makefile:  don't warn about frame sizes for user threads

https://github.com/zultron/linuxcnc/issues/13
rtapi_compat.c:  hardcode kmodule extension and print diagnostics on failure

flavor_t mod_ext is '.so' for userland threads; now hardcoded to
'.ko'.
realtime.in:  fix shmdrv unloading behavior

Don't unload shmdrv if shm segments still exist.  This allows multiple
instances.

Also, during load, bomb out if shmdrv.ko was inserted but doesn't come
online.
.gitignore several bits of build cruft