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Rework char_arg to take the argument instead of an argument list.
Also, rename it to validate_char_argument since it follows the new conventions of the validate* functions.

Refactoring: Change the signature of char_arg() to return a wchar_t instead of NODE*.
This is what all of the callers want, anyway.

This change makes it clear that char_arg() is responsible for freeing whatever it allocates.

Rework char_arg() so that it doesn't modify the input list.
This is part of a wider effort to simplify the code base by removing side effects.

Add additional tests for ASCII to prepare for refactoring of char_arg().

Fix the way the empty string is expressed in some RUNRECOVERABLEERRORTEST tests.
It's currently impossible to put an empty string into a list literal.
What looked like the empty string ended up being ignored.

This change replaces the list literal with a call to LIST, which can create a list that contains the empty string.

Refactoring: rename lbfs to lbutfirsts for consistency with how other logofuncs are named.
Most other primitives are implemented by functions that are named as "l" concatenated to the primitive's full name, not an abbreviation.

Fix an off-by-one error in the translation of a byte buffer to a Unicode string.
This wasn't consequential because make_buffer allocated an extra wchar_t for a NUL-terminator and initialized the buffer to zeros (with calloc) beforehand. Still, it's better for the code
to logically correct in addition to functionally correct.

Fix a comment

Finish fixing Bug #616 (Empty words created by DLLCALL now act like the empty word).

The earlier fix missed that ldllcall() converts the buffers from Unicode to bytes after the function call.
This would have been okay except for two things. First, Null_Word is not a STRING but CASEOBJ and so
getstrptr returns something that was not intended to be a string pointer. Second, there's an off-by-one
in this conversion logic modifies one byte more than the length. This was previously safe because
make_buffer allocates one more character than needed to pad it with a NUL character, but is not safe for
Null_Word where getstrptr returns a pointer to who-knows-what?

The follow-up fix is to skip the Unicode-to-byte translation if the buffer NODE* is Null_Word.

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.