[C latin-unity] Support write-region's kludgy feature

APPROVE latin-unity >>>>> "Ville" writes: Ville> The attached quick patch appears to fix latin-unity's Ville> breaking of calls such as (write-region "" nil Ville> "/tmp/fubar"), which is actually the reason for the Ville> recently reported "apel-1.27 package breaks nnml". OK. The string API of write-region is for use only by programmers, who should be taking care with coding anyway. latin-unity is to protect users from the stuff that Mule does behind their backs, so I see no real problem in disabling it in program-only contexts.