[Mutt] #2952: <BackSpace> should use terminal settings

[Mutt <fleas@xxxxxxxx>]

#2952: <BackSpace> should use terminal settings

 When <BackSpace> is used in a macro or key binding, the expected result is
 that pressing the backspace key will trigger that macro or key binding.
 However, since <BackSpace> is considered an alias for {{{^H}}} and not all
 terminals emit {{{^H}}} when the backspace key is pressed, this can lead
 to unexpected behavior. Mutt *should* attempt to detect what the BackSpace
 key is by fetching the "erase" key from current environment (the same way
 stty does).

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Ticket URL: <http://dev.mutt.org/trac/ticket/2952>