net-snmp: init script fixes

Changes:
1. Add source function and status command.
2. make it possible to change the start arguments with a default file
   in the same way as debian.
3. change the default INITSCRIPT_PARAMS
4. Add PIDFILE and fix restart
5. remove the postrm sicne we use the one from update-rc.d

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