Distinguish <c-h> and Backspace on terminals where that is possible.
Distinguish <c-h> and Backspace on terminals where that is possible.

Different terminals send different codes to indicate backspace, usually one of
\x08 or \x7f, so Kakoune blindly treated both as backspace. However, a given
terminal is only likely to use one of those, and mnemonic control codes like
<c-h> are a precious resource so we should endeavour to keep backspace and
<c-h> separate when we can. Luckily, termios tells us what code our terminal is
currently using, and Kakoune already reads the information at startup, so we can
just use that information.

Thanks to @krobelus for figuring out the C++ syntax required.

Fixes #3863.

alexherbo2/kakoune,mawww/kakoune,jkonecny12/kakoune,casimir/kakoune,lenormf/kakoune,mawww/kakoune,jkonecny12/kakoune,lenormf/kakoune,danr/kakoune,Somasis/kakoune,mawww/kakoune,casimir/kakoune,alexherbo2/kakoune,Somasis/kakoune,danr/kakoune,danr/kakoune,lenormf/kakoune,Somasis/kakoune,casimir/kakoune,Somasis/kakoune,alexherbo2/kakoune,danr/kakoune,mawww/kakoune,casimir/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,jkonecny12/kakoune,lenormf/kakoune