r12986: Use d_fprintf(stderr, ...) for any error message in net.

r12986: Use d_fprintf(stderr, ...) for any error message in net.

All 'usage' messages are still printed to stdout.

Fix some compiler warnings for system() calls where we didn't used the
return code.  Add appropriate error messages and return with the error
code we got from system() or NT_STATUS_UNSUCCESSFUL.
(This used to be commit f650e3bdafc4c6bcd7eb4bcf8b6b885b979919eb)