net-snmp: move persistent dir to /var/lib/net-snmp

Move persistent dir to /var/lib/net-snmp and fix security contexts for
them.

Signed-off-by: Wenzong Fan <wenzong.fan@windriver.com>
Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>