pw_digital_io_linux: Update mock_vfs

Back all MockFile objects by a real (kernel) eventfd. This does a few
things:
* It simplifies mock fd allocation by simply relying on the kernel fd
  allocation.
* It avoids kFakeFdBase for "real" fd disambiguation.
* It allows mock fds to be put into a real epoll instance, and
  subsequently waited upon for events (EPOLLIN only, currently).

This correspondingly adds MockFile::WriteEventfd() and
MockFile::ReadEventfd() methods for making the file readable and
consuming its readable state, respectively.

Finally, this adds MockVfs::mock_read() and corresponding
MockFile::Read().

Change-Id: I80e3f87c20b1baf10370ac2cbd1d34b04f187b55
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/209594
Reviewed-by: Erik Gilling <konkers@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Jonathon Reinhart <jrreinhart@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>