Replace isc_mem_*_aligned(..., alignment) with isc_mem_*x(..., flags)

Previously, the isc_mem_get_aligned() and friends took alignment size as one of the arguments. Replace the specific function with more generic extended variant that now accepts ISC_MEM_ALIGN(alignment) for aligned allocations and ISC_MEM_ZERO for allocations that zeroes the (re-)allocated memory before returning the pointer to the caller.