Move memset() out of PACKET_INITIALIZE() to prepare for porting Tilera mPIPE

The memset() inside PACKET_INITIALIZE() is redundant in some cases and
it is cleaner to do as part of the memory allocation. This simplifies
changes for integrating Tilera mPIPE support because the size of memory
cleared in that case is different from SIZE_OF_PACKET.

For the cases where Packets are directly allocated and then call
PACKET_INITIALIZE() without memset() first, this patch adds memset() calls.

A further change would use GetPacketFromAlloc() directly.