buse.c: conditional defines to work with older nbd versions
buse.h: use distro's nbd.h instead of copy/paste
loopback.c: fix bug in call to buse_main()
loopback.c, busexmp.c: get rid of compiler warnings
buse.c: add assertions to avoid null function ptrs
Merge pull request #1 from petiepooo/bugfixes

Bugfixes