split-string behavior change

[Stephen J. Turnbull]

Drew Adams writes:

 > I'm not suggesting we change anything now, but I'm curious about the change
 > in behavior of split-string from Emacs 21 to 22.

split-string in Emacs 21 was unable to give appropriate results for

    (split-string csv ",")

where CSV is in comma-separated values format, and has leading or
trailing empty fields.  That turns out to be a moderately big deal,
because it is non-trivial to code this function efficiently and
correctly.  (At least, that was the motivation I was told for XEmacs
not removing nulls at all.  Apparently how to parse CSV correctly was
a FAQ in some circles.)  Also, for several cases it gave different
results from XEmacs (including this case).

The current behavior is the result of a compromise which preserves the
magic behavior of removing empty fields in the one-argument case, uses
the simpler behavior of just splitting on the regexp always in the
two-argument case, and restores magic behavior in the three argument
case with a non-nil third argument.