Clean up primitive for reading from file. Never return EOF character.
Stop using null character to indicate EOF as well. Instead, always use a
second product to indicate EOF, and require calls to use it.
For example usage of file operations, see filesystem.mu.
Is it ugly that we don't actually write to disk unless we wait for the
writing routine to exit? Maybe there's a nice way to wrap it. At any
rate, all buffering is explicit, which seems a win compared to *nix.
