Responsive logging

Mostly Windows-specific things:
  - Make the Windows Python Popen workaround disabled by default. Fortunately it does not seem to be needed when emscripten is invoked from the command line via the emcc.bat scripts. Keeping it around behind an .emscripten param/env var EM_POPEN_WORKAROUND in case it is still needed.
  - Unify logging in emscripten.py behind a single function to force flush all logging immediately to console. On Windows, building large codebases would look like they hang, since the above Popen workaround and delayed logging made the build output appear only after the externally spawned processes finish.
  - Add machinery to external process spawns and waits with EM_BUILD_VERBOSE >= 3 to be able to debug where the build hangs, if it hangs.