Added option to display dump/restore stats

Currently criu writes dump/restore statistics to a file. This patch adds
the option '--display-stats' which additionally displays the same
information on the console:

 # criu dump --display-stats -D /tmp/cp -t <PID>
 Displaying dump stats:
 Freezing time: 133 us
 Frozen time: 135173 us
 Memory dump time: 114760 us
 Memory write time: 85107 us
 IRMAP resolve time: 0 us
 Memory pages scanned: 2099 (0x833)
 Memory pages skipped from parent: 0 (0x0)
 Memory pages written: 1052 (0x41c)

 # criu restore --display-stats -D /tmp/cp
 Displaying restore stats:
 Pages compared: 0 (0x0)
 Pages skipped COW: 0 (0x0)
 Pages restored: 51227 (0xc81b)
 Restore time: 57428 us
 Forking time: 51473 us

v2:
 - fix compile failure on 32bit ARM

travis-ci: success for Added option to display dump/restore stats (rev2)
Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>