make(1): split Buf_Destroy into Buf_Done and Buf_DoneData

In all cases except one, the boolean argument to Buf_Destroy was
constant.  Removing that argument by splitting the function into two
separate functions makes the intention clearer on the call site.  It
also removes the possibility for using the return value of Buf_Done,
which would have made no sense.

The function Buf_Done now pairs with Buf_Init, just as in HashTable and
Lst.

Even though Buf_Done is essentially a no-op, it is kept as a function,
both for symmetry with Buf_Init and for clearing the Buffer members
after use (this will be done only in CLEANUP mode, in a follow-up
commit).

(rillig)