Strictly speaking we have to close all fds before reopening them from the
image. This can be done using not yet merged nextfd syscall, but it's not
yet merged :( By now just close std and 255 (bash's) descriptors. This is
ugly, but I prefer to wait for the nextfd syscall.

But skip the init tasks' stds becase the existing zdtm implementation
uses system() in such tasks which doesn't work when all fds are closed.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>