add system_info(ets_limit)

Add system_info(ets_limit) to provide a way to retrieve the runtime's
maximum number of ETS tables. Add tests and documentation for it too.

Also repair the alphabetical order of system_info/1 argument descriptions
in the documentation and in the erlang.erl clauses. Add new preloaded
erlang.erl due to that change. Also ensure all system_info/1 clauses are
represented in the erlang.xml source documentation -- a couple had been
inadvertently dropped in previous commits when other clauses were added.