Add a dissector table for the 802.3 "slow protocols" subtype, split the
dissectors for those protocols into separate files, and have them
register in that dissector table.
Remove some unnecessary #includes while we're at it.
Change-Id: Ic36c9c255efdd348055fa4f21fd6cc094f74e378
Reviewed-on: https://code.wireshark.org/review/1891
Reviewed-by: Guy Harris <guy@alum.mit.edu>