check it for default/lo, test values for config options are randomly
generated and restricted by rand_limit

changes:
v2: make for all config options, avoid use of "system".
v3: err->fail, run in own set of namespaces, add random,
check not all options.
v4: use all options because test is run in net-namespace,
use ARRAY_SIZE macro
v5: check fscanf and snprintf return values properly
Signed-off-by: Pavel Tikhomirov <ptikhomirov@parallels.com>
Acked-by: Andrew Vagin <avagin@odin.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>