shell: commands help unification

1. Changed return value of function: shell_cmd_precheck from bool to
int. Now it returns:
  0       when argument count is correct and help print is not requested
  1       when help was requested and printed
  -EINVAL on wrong arguments count
This change simply shell_cmd_precheck usege in command handlers.

2. Unified all commands in shell_cmd.c file.

3. Fixed a bug where help was not printed on wrong argument count.

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordicsemi.no>
3 files changed