restore: open the pidfile with O_EXCL

There is a potential attack here where if someone is restoring something and
criu write the pid to a file the attacker controls, the attacker can then
re-write that to whatever pid they want. ciru should instead open the file with
O_EXCL so that the restore fails if the file exists.

We don't need O_TRUNC here since we're O_EXCL-ing the file.

Reported-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>