Support multiple files ("FileList" per FTS-1) in subj when creating .REQ files

Previously, this feature assumed just one request per FREQ message, however
FTS-1 make this clear:

      If  one  or more  of FileAttached, FileRequest, or  FileUpdateReq  are
      asserted  in an AttributeWord, the subject{72} field is interpreted as
      a  list of file specifications  which may include wildcards and  other
      system-dependent data.  This list is of the form

      FileList = [ FileSpec { Sep FileSpec } ] Null

      FileSpec = (* implementation dependent file specification.  may
                    not contain Null or any of the characters in Sep. *)

      Sep      = ( " " | "," )  { " " }