Use pcap_snprintf() instead of snprintf().

Use pcap_snprintf() instead of snprintf().

On UN*Xes with snprintf(), we just #define pcap_snprintf to snprintf.

On UN*Xes without snprintf(), we provide our own, but call it
pcap_snprintf().

On Windows, we have a routine that wraps _snprintf(), with C99 semantics
(ensuring null termination if the string won't fit), called
pcap_snprintf(), and use that.