Make TCP options parsing stricter

[ae]

Summary

Rework tcpopts_parse() to be more strict. Add length checks for specific TCP options. The behavior is copied from tcp_input.c.
The main purpose of the change is avoiding of possible out of mbuf's data access.

Reported by: Maxime Villard

[melifaro]

sys/netpfil/ipfw/ip_fw2.c

333

Would help if we add a comment suggesting that the parsing logic mimics one from tcp_dooptions().

334

Since we're talking about safeness: can we change the signature to be const struct tcphdr *tcp?

338

Would it be possible to rename x to something more resembling the remaining byte count?

For example, tcp_dooptions() uses cnt for the similar purpose.

[emaste]

Looks reasonable, with good suggestions from @melifaro

[ae]

Commited in rS355712.