Move most of the `net` collection out of the core.

- Most units and signatures from the `net` collection
  are now in `compatibility-lib`.
- Most of the actual libraries are in the `net-lib`
  package.
Fix uses of 'mzscheme' in comments.
Move some simple modules to `racket/base`.
Start on removing `mzscheme` from the core.

- xform builds without the mzscheme weirdness.
- gets past bootstrapping from source in setup
- finishes make install