shell: refactor default shell command handling

before, we actually duplicated code for handling of shell commands
supplied to shell_init and those hardcoded by MODULE_SHELL_COMMANDS.

This patch refactors shell.c find_handler and print_help so there's
no duplicated code.
shell: fix compile warning and add some comments
Merge pull request #300 from kaspar030/shell_refactor

shell: refactor default shell command handling