split: move parsing outside of *Splitter::new()
split: move parsing outside of *Splitter::new()

Move the parsing of the output chunk size from inside
`ByteSplitter::new()` and `LineSplitter::new()` to outside. This
eliminates duplicate code and reduces the responsibilities of the
`ByteSplitter` and `LineSplitter` implementations.

uutils/coreutils,uutils/coreutils,uutils/coreutils