Create subdissector table for RTAC Serial and have dissectors register with it, rather than have the RTAC Serial dissector go find all of the dissectors its interested in.

Change-Id: I6b6a05ec242e4798fb56ffa43c661ec277aca955
Reviewed-on: https://code.wireshark.org/review/3984
Reviewed-by: Chris Bontje <cbontje@gmail.com>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>