files: Create fake name for remapped file if original name is busy

When live-migrating a container on NFS the link-remap cannot
create original file on target machine, since the file name
is still visible -- source CT is alive. The proper fix would
be to move file aside, then create link, then move original
file back, but since NFS changes file name anyway, we just
do the same -- generate another name for link.

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