Use designated initializers instead of memset()/MEM_ZERO for structs

Use designated initializers instead of memset()/MEM_ZERO for structs

In several places, the structures were cleaned with memset(...)) and
thus the semantic patch converted the isc_mem_get(...) to
isc_mem_getx(..., ISC_MEM_ZERO).  Use the designated initializer to
initialized the structures instead of zeroing the memory with
ISC_MEM_ZERO flag as this better matches the intended purpose.