[Libguestfs] [PATCH nbdkit v3] common: Move shared bitmap code to a common library.

[Richard W.M. Jones]

v2:
https://www.redhat.com/archives/libguestfs/2018-December/msg00039.html

v2 -> v3:

 - Fix all the issues raised in Eric's review.

 - Precompute some numbers to make the calculations easier.

 - Calculations now use bitshifts and masks in preference to division
   and modulo.

 - Clear existing bits before setting (which fixes a bug in the cache
   filter).

Rich.