src/generator: Move all logic into libostree-1.so

src/generator: Move all logic into libostree-1.so

This pushes down the code for parsing the `ostree=` cmdline
in the generator into code that's part of libostree-1.so.

This is prep for using logic shared in libotcore.la.

But in general it's just cleaner to also keep the binary
entrypoint to just be a trampoline into the C library.