sandbox: use waitpid consistently

The BMM PNaCl toolchain doesn't currently support waitid.  Arguably it
should because it's in POSIX, but the userland/Linux signal ABI
mismatch makes it non-trivial because of the siginfo_t.  Since we
don't need the siginfo_t anyway, we can just use waitpid instead.

While here, use WIFEXITED and WEXITSTATUS for interpreting status
instead of directly comparing it to the expected exit code.

Review URL: https://codereview.chromium.org/928963003

Cr-Commit-Position: refs/heads/master@{#317080}

sandbox/linux/services/credentials.cc

@@ -114,7 +114,7 @@ bool ChrootToSafeEmptyDir() {
   int status = -1;
   PCHECK(HANDLE_EINTR(waitpid(pid, &status, 0)) == pid);
 
-  return kExitSuccess == status;
+  return WIFEXITED(status) && WEXITSTATUS(status) == kExitSuccess;
 }
 
 // CHECK() that an attempt to move to a new user namespace raised an expected
@@ -174,12 +174,14 @@ bool Credentials::CanCreateProcessInNewUserNS() {
   // have disappeared. Make sure to not do anything in the child, as this is a
   // fragile execution environment.
   if (pid == 0) {
-    _exit(0);
+    _exit(kExitSuccess);
   }
 
   // Always reap the child.
-  siginfo_t infop;
-  PCHECK(0 == HANDLE_EINTR(waitid(P_PID, pid, &infop, WEXITED)));
+  int status = -1;
+  PCHECK(HANDLE_EINTR(waitpid(pid, &status, 0)) == pid);
+  CHECK(WIFEXITED(status));
+  CHECK_EQ(kExitSuccess, WEXITSTATUS(status));
 
   // clone(2) succeeded, we can use CLONE_NEWUSER.
   return true;