An alternative to #3366 which added a dupcol keyword.

This one doesn't deprecate makeunique but overloads it instead with three kinds of values allowed:

1. true/false for make a new column or raise an error (current behaviour),
2. a Function of two parameters that determines how to combine duplicate column values into one, or
3. one of a set of keywords that handle some of 1 and 2

The currently allowed keywords are:

• :makeunique – create a new uniquely named column
• :error – raise an Error
• :update – update the left-hand column with non-missing values from the right
• :ignore – ignore the duplicated column

The implementation is simpler than #3366 and is more flexible. It doesn't address the desire to have a function that would determine how makeunique=true would generate new column names, but that could be accomplished by adding a new keyword argument.