wait4 syscall should return pid (tgid) and not the tid to userspace

This is what the kernel does. Fix emulation to do the same.

In glibc waitpid() i.e. __waitpid() is implemented in terms of wait4()
i.e. __wait4() which is a thin wrapper over the wait4 syscall. wait4
syscall returns the pid (tgid in kernel parlance) and not the thread id
(tid). tid and pid will, of course, often be different in multithreaded
situations.  See also `man wait4` and `man waitpid`.

This should fix the unexpected behavior of waitpid() in programs
in which the return pid_t is checked against an expected value or used
for further bookkeeping/comparisons. In such programs, the failure
can be rare because due to thread sequencing, sometimes the tid
happens to be numerically the pid that is expected and sometimes it is
not.

src/record_syscall.cc

@@ -7104,7 +7104,7 @@ static void rec_process_syscall_arch(RecordTask* t,
       if (tracee) {
         // Finish emulation of ptrace result or stop-signal
         Registers r = t->regs();
-        r.set_syscall_result(syscallno == Arch::waitid ? 0 : tracee->tid);
+        r.set_syscall_result(syscallno == Arch::waitid ? 0 : tracee->tgid());
         t->set_regs(r);
         if (syscallno == Arch::waitid) {
           remote_ptr<typename Arch::siginfo_t> sip = r.arg3();