* dtable.cc (dtable::fixup_close): Define new function.

* dtable.cc (dtable::fixup_close): Define new function.
(dtable::fixup_after_exec): Use fixup_close() and detect when it was not
possible to open an inherited file handle.
(dtable::fixup_after_fork): Defensively close any file handles which were not,
for some reason, inheritable.
* dtable.h: Make #pragma once.
(dtable::fixup_close): Declare new function.
* fhandler_console.cc (fhandler_console::set_unit): Set I/O handles to NULL
when this function fails.