policycoreutils: sandbox: trap sigterm to make sure

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This patch looks good to me. acked.
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iEYEARECAAYFAk5WtTkACgkQrlYvE4MpobOnzACfU3UZN1x4c5edKq2v/CheBgPc Vd4An0xpWJG6zxPtLOpKadGB+/AV8mhz
=gG+X
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