Revert "shell: remove useless copying of input string"

This reverts commit 33239487bf8f7f85a7d27ad5bd912079c503b4cd.

indeed, this commit breaks some shell handler which use strtok to tokenize the input string.
strtok returns always a nullptr.

DO NOT MERGE: https://github.com/RIOT-OS/RIOT/pull/707 will come with a clean solution