Migrate to bstring fork

[rdmark]

We're currently using a modified bstrlib, based on the original code by Paul Hsieh.

There is contemporaneous fork called bstring which has improvements such as a build system, tests, and memory safety. It claims binary compatibility with bstrlib.

https://github.com/msteinert/bstring

The two projects had some parallel developments that should be harmonized.

At the same time, it might be worth experimenting with having bstring as a submodule, built as a separate static library linked with each binary that needs it. Instead of having it part of the libatalk library.