We need the current thread and the trace internal state so that we can
use the fd beautifier to augment syscall returns, so use struct
syscall_arg with some fields that make sense on returns (val, thread,
trace).

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-lqag8e86ybidrh5zpqne05ov@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>