Remove unsafe ScopedSigaction
Remove unsafe ScopedSigaction

ScopedSigaction is not async-signal-safe, so it cannot be used after fork.
Replace it with plain sigaction, which is safe. This is in a unique child
anyways, so it doesn't need any cleanup.

PiperOrigin-RevId: 237102411
Change-Id: I5c6ea373bbac67b9c4db204ceb1db62d338d9178

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor