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Fix Bug #616; Empty words created by DLLCALL now act like the empty word.

I chose to fix this in DLLCALL, making make_buffer return Null_Word like make_strnode does.

An alternative fix could have been to find all places which assume that Null_Word is the only
empty word and fix those to look at the string's length. That would have been a more robust
fix, but it also would have been more invasive and undone whatever the original authors were
tying to accomplish by making Null_Word a singleton.

Update the expected test log to include the output of some new tests, as well as some bug fixes.

Cleanup: Remove the special case in butfirst() and llbutlast() to return Null_Word.
make_strnode already has this logic, so there's no need to duplicate it.

Add tests that FIRSTS, BUTFIRSTS, and BUTLAST take ownership of any node :erract provides them.

Fix signficant regressions in LAST introduced by recent refactoring.

Document the calling convention of logofunc, as I understand it.

Fix a recent regression in FIRST where it could leak a value provided by :erract.

Fix a recent regression in BUTFIRST where it could return a freed NODE*.
Calling gcref(arg) could inadvertantly free val if it were a list.
This change also adds some more comments and an assert around the memory
management contact of validate_argument.

Change BUTFIRSTS so that, on bad input, it considers its entire input bad, not just one element.
This is consistent with how other procedures work.
The logic was adpated from lfirst() and lfirsts().

Change FIRSTS so that, on bad input, the entire input is replaced instead of the bad element.
This is consistent with how other procedures work.

I had thought the code would use validate_argument before entering the
parsing loop, but I didn't like the idea of making two passes over
the list. Instead, the code optimistically collects the output and,
on error, cleans up and gets a whole new input.

Overall, the code didn't turn out clean as I had hoped. Still, I
prefer the new behavior, so I'm going to keep it, and probably apply
similar code to BUTFIRSTS.