9p: fix version detection, fix dissect_9P retval

Fix version detection (detect against full string instead of prefix),
properly dissect Tcreate extension field (9p2000.u only), dissect
Tunlinkat flags (9p2000.L).

Refactor pattern to dissect string[s] types for DRY.

Convert to use tcp_dissect_pdus. I have not seen a fragmented case, but
maybe that may happen in the future.

The main motivation for touching 9p was that it returns bogus values
for some types. This has been fixed by properly increasing offset, and
always return the captured length.

Change-Id: If2184204ae9c853b94aca8ade3763d7fe523fa86
Reviewed-on: https://code.wireshark.org/review/2836
Reviewed-by: Christopher Maynard <Christopher.Maynard@gtech.com>
Reviewed-by: Evan Huus <eapache@gmail.com>
Reset state when cycling ring-buffer files in tshark.

This has several implications:
 - we match user expectations that a ring-buffered tshark capture will run
   forever without running out of resources (except where we still have leaks)
 - we lose reassembly and request/response matching when the relevant packets
   are split across files, but this actually makes our output more consistent
   with dissecting those files after-the-fact

I have not made it configurable in this change because I'm not really sure
there's a use case for the old behaviour - if you're running a ring-buffer
capture in the first place it's because you're willing to discard old data to
limit resource usage. If you want the full dissection without breaks, just don't
use a ring buffer at all and take the resource hit in both disk and memory.

Change-Id: I7d8f84b2e6040b430b7112a45538041f2c30f489
Reviewed-on: https://code.wireshark.org/review/2669
Reviewed-by: Jörg Mayer <jmayer@loplof.de>
Reviewed-by: Evan Huus <eapache@gmail.com>