Implementation of igmp_set/get

* I changed the sw_api. There were two functions: igmp_enable
and igmp_disable which were using the same ioctl command
(SWCFG_SETIGMP) so I united them in one function igmp_set
This receives as its third argument a flag which decides
whether the IGMP snooping should be set on or off
* the test function will get only the global setting

Signed-off-by: Claudiu Ghioc <claudiu.ghioc@gmail.com>