make __set_open_fd() set cloexec state as well

->close_on_exec[] state is maintained only for opened descriptors;
as the result, anything that marks a descriptor opened has to
set its cloexec state explicitly.

As the result, all calls of __set_open_fd() are followed by
__set_close_on_exec(); might as well fold it into __set_open_fd()
so that cloexec state is defined as soon as the descriptor is
marked opened.

[braino fix folded]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>