In a noninteractive shell, asynchronous commands ignore SIGINT and
SIGQUIT. We typically use such shells to feed fifo buffers which we
do want to cancel them on Control-C. Make it so.
Same for SIGQUIT; that one is not typically used but I expect that
along the Kakoune server it kills any child processes that (haven't
been daemonized).
Note that for unknown reasons, Bash already doesn't ignore SIGINT in
async processes that use "eval".
Note that Dash has a bug that prevents this from working;
proposed fix is at
https://lore.kernel.org/dash/20240329153905.154792-2-aclopte@gmail.com/
(While at it balance out some parens, to help the m command)
The off-by-one was introduced by cd9b1e66 which changed `column-1` to `column`.
The refactoring solves some esoteric quoting errors: I think cases like
unbalanced braces in the bufname and client were not supported.
