Remove insane *_FILENO and EXIT_* usage

Remove insane *_FILENO and EXIT_* usage

Any system having different assignments than the usual 0, 1, 2 for
the standard file numbers and 0, 1 for the exit-statuses is broken
beyond repair.
Let's keep it simple and just use the numbers, no reason to fall
out of the window here and bend down for POSIX.
In one occasion, the ret-variable was not necessary. The check was
rewritten.

Signed-off-by: Christoph Lohmann <20h@r-36.net>