#1244 Fix to memory leak and close sockets while reconfiguring snmpd with signal HUP

[Harsha Shivanna]

Bug Description:
Memory and sockets allocated while creating notifications is not freed when snmpd is reconfigured(by sending -HUP signal).

Bug fix details:
register for SNMPD_CALLBACK_PRE_UPDATE_CONFIG callback function for in init functions snmpTargetAddrTable and snmpTargetParamsTable so that memory and sockets can be freed while reconfiguring.

Testing:
Tested on debian paltfrom by reconfiguring snmpd 6 times with 255 hosts. all the functionality of snmpd, walk, set and traps was intact