client/test/maxadmin_test.sh: added new cases for testing maxadmin
service.c:dListListeners:fixed NULL-pointer reference
debugcli.h: replaced magic number 80 with constant integer.
cli.c:execute:fixed command buffer overflow
debugcmd.c:convert_arg:restored return value to 0 in case of error
In order to be able to protect the interface so that mistyped argument
do not endanger MaxScale yet still allow the developer access a router
option has been added that gives the debugcli two modes.
