cgconfig: change function usage behavior
cgconfig: change function usage behavior

remove exit from function usage and add a structure like usage function
in the other tools.
This is suggested by Ivana Hutarova Varekova <2e3918a0a366ea7e658a2062ab582c898a7e1b1b@redhat.com>

now usage willn't exit, so we should help it in err case.

v4:
 - fix some code style reported by checkpatch.pl

Signed-off-by: Libo Chen <3ddb76ce901f50183fdff8d1ff6862f91b38880c@huawei.com>
Acked-by: Ivana Hutarova Varekova <2e3918a0a366ea7e658a2062ab582c898a7e1b1b@redhat.com>

rafalmiel/libcg,rafalmiel/libcg,rafalmiel/libcg,rafalmiel/libcg