Consistently use defined exit codes.

Consistently use defined exit codes.

Tools have been inconsistently using exit codes when started with
--help/--version.
With this patch all tools use exit codes E_USAGE/E_VERSION defined
in net-support.h.

There's probably no rule what exit codes should programs use when
started with --help/--version, but when I'm checking programs on
my system most of them use 0 (SUCCESS) exit code.
Therefore I redefined E_USAGE/E_VERSION to EXIT_SUCCESS (0).