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}

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