Bugfixes

With these changes, BUSE compiles on older and current ubuntu dists without warning.  There are 5 separate commits, each adding a specific capability.
1) (36a70c0): conditional defines around newer nbd macros (eg. not in 3.0.0 kernel)
2) (8439322): uses mainline's nbd.h instead of pasted content from nbd.h in buse.h
3) (c322c10): loopback wouldn't work due to using argv[1] for both ends
4) (e4fd3b7): eliminates unused variable compiler warnings in samples
5) (6ba3d26): prevents calls to null function pointer for unimplemented operations