cmd/split: add a new --keep-boundaries option.

cmd/split: add a new --keep-boundaries option.

If you provide multiple input files on the command line, sometimes you want
to merge them togther into a single file before re-chunking them (the
default).  But sometimes you want all the files to be treated separately for
chunking purposes, ie. when you know that some of the files will never
change so there's never any point in merging it with previous/subsequent
files.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>