While adding usage information, 44d742e (perf list: Add usage,
2013-10-30) broke
$ perf list --raw-dump
by asking parse_options() to stop at
non-option (PARSE_OPT_STOP_AT_NON_OPTION), hence parsing the option
--raw-dump and barfing. Ask it to keep unknown options
unparsed (PARSE_OPT_KEEP_UNKNOWN) so that we have a chance to handle it
later. The trade-off that this patch makes is that
$ perf list --foo
will not error out like it used to.