util: Add "/bin" to PATH when spawning helpers

We call tar, ip, iptables, etc. when restoring container.
The problem is that these stuff is called from inside new
mount namespace after pivot_root(). But the execvp uses
PATH variable inherited from the host system, which may
not reflect real binaries layout.

Add "/bin" to path as temporary workaround.

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