Script for generating mcast stats is moved from common to platform
specific files. The logic for generating mcast stats on non broadcom
platforms is different and the stats format is also different.
For generating stats we are are sending SIGUSR1 signal to mcproxy
process which then writes multicast stats to /tmp/igmp_snooping_stats.
service_stop is a legacy function and it should never be used with
procd services which are automatically stopped by /etc/rc.common
Also remove unnecessary boot() which just runs the "start" function.
