Change the last two args of split/rsplit to 'limit' and 'keep' keywords

Fixes #6412.

cc: @StefanKarpinski 

Another change that would probably go along with this is having `replace(str, pat, replacement; limit=0)` 
Ok, should be fixed now. As a side note, we should really go through and standardize the function API some more. I noticed several various ways of documenting while looking over base.rst.