get exdir, checkdir on it:  create or not, as determined earlier;
  assumed to be correct for system (VMS:  convert to lowercase if
  not already; check in [x.y]z.dir;1 format, but change to [x.y.z.
  format for appending other sections--mapname will be responsible
  for getting it, appending path and name, before returning)

mapname():
  checkdir( ..., INIT)     -->  malloc space to put path as built; add exdir
         (d:/tmp/)                         (disk:[tmp.)
  checkdir(path component, COMPONENT | CREATEDIR) --> map as required?
         (d:/tmp/unzip/)                   (disk:[tmp.unzip.)
         (d:/tmp/unzip/jj/)                (disk:[tmp.unzip.jj.)
         (d:/tmp/unzip/jj/temp/)           (disk:[tmp.unzip.jj.temp.)

         (d:/tmp/unzip/jj/temp/msg.outdir) (disk:[tmp.unzip.jj.temp]msg.outdir)
