GN_Fpb, Push back a character into filter

IN:
     IX = filter handle

OUT, if call successful:
     Fc = 0

OUT, if call failed:
     Fc = 1
     A = error code:
          RC_HAND ($08), bad filter handle
          RC_PUSH ($0E), push back error
               1. never read from filter
               2. Already pushed back one character
          RC_EOF ($09), filter is empty

Registers changed after return:
     ..BCDEHL/IXIY same
     AF....../.... different