fix shell use of strtok

Pass the pointer to the remaining tokens to handler instead. The original string has already been used.

A result of this is that shell commands **should** use `strtok` now. I've included exemplary rewrites of the shell commands in sys/shell/commands.
It is still possible to parse the parameter without the use of `strtok` due to its implementation - it only inserts `\0` and `strtok_r` returns a pointer to the following character. I've changed `sc_rtc.c` to reflect this.

Do we want this?
I see no reason not to, but it needs adjustment of the remaining shell commands.
If we want we could also pass the command name token to the handler in order to make printing usage information easier and allow for different names for the same command.