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`.
Print error message on any quoting error
Convert examples
Do not skip over the null terminator
shell: adapt register prefix command to new shell api
Fix _settime_handler
Merge pull request #3 from mehlis/shell-args-prefix

shell: adapt register prefix command to new shell api
Merge pull request #760 from Kijewski/shell-args

shell: Use argc and argv in shell handlers