User namespace

Implementing user namespace support in CRIU requires a few changes on kernel side. First of all when a new user namespace is created its capability get dropped off, thus any kernel aspect guarded with capable() may fail. In particular we found that the following things are not functional when we restore tasks running in own user namespace:

- setup of memory descriptor (prctl syscall)
- access to /proc/pid/map_files which we use for restore of shared memory
- restoration of SO_RCVBUFFORCE socket option

The list will be updated with time. To resolve the problems we need to address every issue and modify the kernel. For first bullet there is a patch already floating around in kernel mailing list.