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 branch 'bugfixes'
apply GPLv2 license to BUSE project (original author must approve)
Merge pull request #1 from petiepooo/gplv2-lic

apply GPLv2 license to BUSE project (original author must approve)
Merge pull request #2 from petiepooo/gplv2-lic

apply GPLv2 license to BUSE project (original author must approve)
Merge pull request #1 from petiepooo/bugfixes

Bugfixes
Merge remote-tracking branch 'origin/master'
Merge branch 'master' of https://github.com/acozzette/BUSE