  >< B - B A C K S P A C E   A V A I L A B I L I T Y   O R    V A L U E ><

Can either tell the editor that the terminal hasn't or has (the default) a
backspace function i.e. a character that will move the cursor back 1 (a few
older terminals don't), or can tell Q what the backspace character is.

Format
 B
 B <yes/no>
 B <char>
 <yes/no> any of "Y","N","T","F" (yes,no,true,false) with the obvious meanings.
 <char>   The backspace character, as 3-digit octal
 Both of the above are optional. If no parameter is given, ".NOT.current state"
is assumed i.e. the logical variable set by "B <yes/no>" is inverted.

Examples
 B     /* Inverts availability: if the editor did assume ^H worked it won't now
 B NO  /* Tells editor definetily no backspace function
 B 10  /* Tells editor backspaces is ^H, but says nothing about whether it's
       /* available on this terminal (the logical variable accessed by
       /* "B <yes/no>" is not altered).
