Switch one's complement checksum implementation

Borrows the `in_cksum` code from tcpdump, which borrowed from FreeBSD.
It handles unaligned data better and also unrolls the inner loop to
process 16 two-byte values at a time versus 2 one-byte values at a time
in the previous version.  Generally measured as ~1.5x faster in a
release build.  The new API should generally be more amenable to any
future optimization explorations since all relevant data blocks are
available within a single call rather than spread across multiple.

src/analyzer/protocol/tcp/TCP.h

@@ -95,7 +95,7 @@ protected:
 	// Returns true if the checksum is valid, false if not (and in which
 	// case also updates the status history of the endpoint).
 	bool ValidateChecksum(const struct tcphdr* tp, TCP_Endpoint* endpoint,
-				int len, int caplen);
+				int len, int caplen, bool ipv4);
 
 	void SetPartialStatus(TCP_Flags flags, bool is_orig);