Add support for variable TCP-header options

[zeeshanlakhani]

Background

As per IETF RFC #793's Options field:

Options may occupy space at the end of the TCP header and are a multiple of 8 bits in length. All options are included in the checksum. An option may begin on any octet boundary. There are two cases for the format of an option:

• Case 1: A single octet of option-kind.
• Case 2: An octet of option-kind, an octet of option-length, and the actual option-data octets.

The option-length counts the two octets of option-kind and option-length as well as the option-data octets.

Note that the list of options may be shorter than the data offset field might imply. The content of the header beyond the End-of-Option option must be header padding (i.e., zero).

Currently defined options include (kind indicated in octal):

Kind	Length	Meaning
0	-	End of option list
1	-	No-Operation
2	4	Maximum Segment Size

More about these definitions can be found in the spec linked.

Currently, we've left this as a TODO in working with the data offset portion of the TCP header.

Solution?

In dealing with parsing dynamic options, we've done similar work w/ NDP options, but we need to figure out what the best fit is here for handling these specific options from the spec.