[Libguestfs] [PATCH nbdkit 5/9 patch split 1/5] Create libnbdkit.so.

This is the previous 5/9 patch posted earlier today, split into reviewable chunks. This passes bisection with -x 'make && make check', but I didn't work very hard on the commit messages, so I refer you back to the original patch to explain how it works: https://www.redhat.com/archives/libguestfs/2020-March/msg00248.html Rich.