Exit with an appropriate error code after printing a usage message: when invoked as ``mclient -?`` or ``mclient --help``, the exit code should be 0, else non-zero. This fixes bug 2956574.
Exit with an appropriate error code after printing a usage message:
when invoked as ``mclient -?`` or ``mclient --help``, the exit code
should be 0, else non-zero.
This fixes bug 2956574.

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb