+ Extract the bulk of sim_tape_wrgap to a new local
   tape_wrgapf (UNIT *uptr, t_mtrlnt *bc) routine, where "bc" is the
   gap length byte count needed.

 + Add a new t_stat sim_tape_errecf (UNIT *uptr, t_mtrlnt bc) to erase 
   a record forward.  "bc" is the RECORD LENGTH; the actual gap includes
   the two metadatum markers present for the record.  Handles rounding
   to ensure a valid erasure (even byte length).

 + Add a new t_stat sim_tape_errecr (UNIT *uptr, t_mtrlnt bc) to erase 
   a record backward.  HOWEVER, this is simple only in the case of an
   erase that exactly matches the record length of an existing record
   immediately prior.  Otherwise, it needs a general scan-and allocate
   routine that can truncate existing records, modeled after the current
   forward erase routine.

 - Modify sim_tape_errecf/r to take a zero record length as meaning
   "erase the tape mark" forward or backward.  Bob seems to want an error
   if the next/previous metadatum is not a tape mark.