selfm: Dissector fails to detect message length on multi-message packets

selfm: Dissector fails to detect message length on multi-message packets

- correction to get_selfm_length function
- fix Fast Message response decoding when multiple auto-configuration sessions are present in capture
- remove depreciated selfm 'simple' dissection

Bug: 11935
Change-Id: I685dba3bddfc61e0d2faf7b0258cd5572ce4e7ed
Reviewed-on: https://code.wireshark.org/review/12888
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>