JSON: Add support for '+++' signals

There are only 3 kind of '+++' signals in strace.
Note: Currently do NOT support the '<unfinished...>/<resumed>' in
JSON format. And this will make the those JSON lines invalid(or you can
just ignore these lines). Need to find a way to solve this problem.

* process.c(sys_exit): slightly reformat the code.
* syscall.c(trace_syscall_exiting): slightly reformat the code.
* strace.c(printleader, trace):
Add support for 'superseded', 'killed' and 'exited' signals.