monitor: refine parse_cmdline()

Since this function will be used by help_cmd() later, so improve
it to make it more generic and easier to use. free_cmdline_args()
is added too as paired function to free the result.

One change of this function is that, when the valid args in input
exceed the limit of MAX_ARGS, it fails now, instead of return with
MAX_ARGS of parsed args in old code. This should not impact much
since it is rare that user input many args in monitor's "help" and
auto complete scenario.

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
