Rewrite unix file opening code to be really careful about what kinds

Rewrite unix file opening code to be really careful about what kinds
of files can be opened, and how.  As of now, only regular files and
character special files are allowed; symbolic links are detected when
opening output files and deleted, thus overwriting the link rather
than writing through it.

Implement new primitive `file-flush-output' which is needed since we
changed file I/O to use buffered stdio some time ago.