build_keep_args: Build a list of arguments to keep
Description
In the process of building data frames of arguments to FUN,
we need to decide which arguments to keep from each source,
..., .dat, and defaults to FUN.
This function does that work in one place.
Usage
build_keep_args(where_to_find_args)
Value
A list with names .dat, dots, and FUN which
gives items to keep from each source.