* fhandle.h (fhandler_pipe::create_guard): Revert change which eliminated

* fhandle.h (fhandler_pipe::create_guard): Revert change which eliminated
SECURITY_ATTRIBUTES argument.
* pipe.cc (fhandler_pipe::open): Duplicate guard from other process and protect
it appropriately.  Eliminate unneeded writepipe_exists temporary variable.  Set
inheritance appropriately.
(fhandler_pipe::set_close_on_exec): Revert change which eliminated handling
guard inheritance.
(fhandler_pipe::fixup_after_fork): Ditto.  Use correct name of entity being
checked by fork_fixup.
(fhandler_pipe::fixup_after_exec): Don't bother with guard here.
(fhandler_pipe::dup): Cosmetic changes and revert creation of writepipe_exists
as noninheritable.
(fhandler_pipe::create): Revert change which eliminated SECURITY_ATTRIBUTES
argument.  Revert change which always made writepipe_exists noninheritable.