stream: add TCP urgent handling options

stream: add TCP urgent handling options

TCP urgent handling is a complex topic due to conflicting RFCs and
implementations.

Until now the URG flag and urgent pointer values were simply ignored,
leading to an effective "inline" processing of urgent data. Many
implementations however, do not default to this behavior.

Many actual implementations use the urgent mechanism to send 1 byte of
data out of band to the application.

Complicating the matter is that the way the urgent logic is handled is
generally configurable both of the OS and the app level. So from the
network it is impossible to know with confidence what the settings are.

This patch adds the following policies:

`stream.reassembly.urgent.policy`:

- drop: drop URG packets before they affect the stream engine

- inline: ignore the urgent pointer and process all data inline

- oob (out of band): treat the last byte as out of band

- gap: skip the last byte, but do no adjust sequence offsets, leading to
       gaps in the data

For the `oob` option, tracking of a sequence number offset is required,
as the OOB data does "consume" sequence number space. This is limited to
64k. For this reason, there is a second policy:

`stream.reassembly.urgent.oob-limit-policy`:

- drop: drop URG packets before they affect the stream engine

- inline: ignore the urgent pointer and process all data inline

- gap: skip the last byte, but do no adjust sequence offsets, leading to
       gaps in the data

Bug: #7411.