shell: Use argc and argv in shell handlers

Compare #708.

Now the tokenization of an input line is done by the shell itself. You
may quote arguments with `"..."`. Empty arguments, supplied by `""` are
preserved. Spaces in between arguments are squasheds; spaces inside
quotes are preserved.

You cannot partially quote an argument. You must not use
- `cmd "abc`,
- `cmd abc"def"`, or
- `cmd "abc"def`.