crtools: also print the current kernel version

In addition to writing the CRIU version to the log file this adds the
current kernel version to the log file:

(00.000008) Version: 3.5 (gitid v3.5-511-ga8cc6cf)
(00.000303) Running on node01 Linux 3.10.0-513.el7.x86_64 #1 SMP Tue Feb 29 06:78:90 EST 2017 x86_64

v2:
 - small changes as suggested by Dmitry (thanks)

Signed-off-by: Adrian Reber <areber@redhat.com>
Reviewed-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>