perf trace: Allow syscall arg formatters to request non suppression of zeros

The 'perf trace' tool is suppressing args set to zero, with the
exception of string tables (strarrays), which are kinda like enums, i.e.
we have maps to go from numbers to strings.

But the 'cmd' fcntl arg requires more specialized treatment, as its
value will regulate if the next fcntl syscall arg, 'arg', should be
ignored (not used) and also how to format the syscall return (fd, file
flags, etc), so add a 'show_zero" bool to struct syscall_arg_fmt, to
regulate this more explicitely.

Will be used in a following patch with fcntl, here is just the
mechanism.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-all738jctxets8ffyizp5lzo@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>