FILEIO: convert flxoverlay to XFILE.

Haven't converted push_flx_overlays as it calls jwrite_chunk.
(fc.type < lastdone) == (fc.type < 0) is always false.
FILEIO: convert font to XFILE.
FILEIO: convert macro to XFILE.
FILEIO: convert picdrive to XFILE.
FILEIO: convert redo to XFILE.
FILEIO: convert tween to XFILE.