The x11-terminal command spawns a potentially long-lived terminal
process. The terminal can is completely independent of the Kakoune
session that created it.
Due to how it's implemented, the spawned terminals will have
environment variables "kak_opt_termcmd" and "kak_quoted_reg_a"
set. This can be surprising, especially since, by convention, the
environment contains no lowercase variables. Let's stop exporting them.
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