Remove fd checks

Allow calling alcove functions that accept a raw fd with any fd. For
safety, alcove disallowed using low fd's (STDIN, STDOUT, STDERR) and
"well known" fd's used by alcove (the child end of the control fd).

However, enforcement was inconsistent because:

* the process could manipulate the fd's used to manage the child procs
  (the parent side of the child's stdin, stdout, stderr)

* the port process may have been re-exec'ed with open file descriptors,
  for example, by valgrind