Added AUFS support.

The AUFS support code handles the "bad" information that we get from
the kernel in /proc/<pid>/map_files and /proc/<pid>/mountinfo files.
For details see comments in sysfs_parse.c.

The main motivation for this work was dumping and restoring Docker
containers which by default use the AUFS graph driver.  For dump,
--aufs-root <container_root> should be added to the command line options.
For restore, there is no need for AUFS-specific command line options
but the container's AUFS filesystem should already be set up before
calling criu restore.

[ xemul: With AUFS files sometimes, in particular -- in case of a
  mapping of an executable file (likekely the one created at elf load),
  in the /proc/pid/map_files/xxx link target we see not the path
  by which the file is seen in AUFS, but the path by which AUFS
  accesses this file from one of its "branches". In order to fix
  the path we get the info about branches from sysfs and when we
  meet such a file, we cut the branch part of the path. ]

Signed-off-by: Saied Kazemi <saied@google.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>