make(1): make modifier handling simpler

Implementing a modifier such as :S or :M should not be concerned with
separating the words of the resulting string.  Ideally this should be
done in the same way by all modifiers.

Before, the :R (filename root) modifier added a separator even if the
resulting filename root was an empty string.  The chances that this
change in behavior breaks anything are epsilon.

The :@ modifier, if it appeared after a :ts modifier, did not use the
word separator from the :ts modifier (which all other modifiers do) but
always added a space.  This behavior has been preserved for now.  It's an
unnecessary inconsistency though.

In contrast to Buffer, the newly added SepBuf uses size_t for memory
sizes and also uses the conventional parameter order (mem, memsize)
instead of the unusual (memsize, mem).

(rillig)