s7comm: Add dissector for S7 Communication

S7 Communication is a Siemens proprietary protocol that runs between
programmable logic controllers (PLC) of the Siemens S7-300/400 family.

Dissector T.125 has to be disabled to let this dissector work.

Change-Id: I578cf270a4ae567f8e20dbabec1ce1e13fc08e6e
Reviewed-on: https://code.wireshark.org/review/3777
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>