Handle SIGTERM as a graceful exit, similar to the `:kill!` command
Handle SIGTERM as a graceful exit, similar to the `:kill!` command

Do not backup modified files, go through graceful destruction of
singletons.

Fixes #3528

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