rc tools menu: work around ARG_MAX limit

[krobelus]

when "menu" arguments exceed ~200k bytes, I get "execve failed:
Argument list too long" (even though ARG_MAX is 2MB).

Reproduce with

evaluate-commands %exp{
	menu asdf %sh{dd 2>/dev/null if=/dev/zero bs=1000 count=200 | sed s/././g}
}

I hit this with rust-analyzer which can send ~70 code actions to
select from, each with a lengthy JSON object, so the :menu invocation
can sometimes reach the effective limit. It can also become slow
(0.5 seconds), maybe because we fork multiple times per argument.

Fix this by passing arguments through $kak_response_fifo.
The on-accept and on-change callbacks have the same problem (sh -c
string too long), so move them to temporary files. This means we
can get rid of some escaping.

Note that there is currently no dedicated way to stop Kakoune from
passing "$@" to the shell process, so define an extra command that
doesn't take args.

Since we serialize arguments into a single string (using "echo
-quoting"), we need to deserialize them before doing anything else.
We currently don't have an off-the-shelf solution for this. Perhaps
this is an argument for "echo -quoting json" (which has been suggested
before I believe).