multicall: add --list to list all utils

[tertsdiepraam]

Closes #6250

I've implemented it more or less like the busybox version now, by just matching on the string --list. Ultimately, we might want to go for more sophisticated argument parsing in the multicall binary.

It is a bit strange though, because all the other things that the multicall binary can do are implemented as subcommands. In other words, one of these is not like the others:

coreutils completion
coreutils manpage
coreutils --list

I think using -- makes sense to clearly separate it from the utils. Maybe completion and manpage should be prefixed with -- too?

[sylvestre]

is that testable ? :)
without a test, it feels fragile

[tertsdiepraam]

Maybe, I'll look into that later today.

[BenWiederhake]

Related to testability: #6198