Merge changes I45940d2d,I100b6246,Ie0ba58b3,I7943e7a6,I172cef38, ...

* changes:
  Remove signal mask checks from native bridge tests.
  sigchain: fix sigaction(signal, nullptr, &sa).
  sigchain: make SIG_DFL handling less broken.
  sigchain: allow use of sigprocmask inside handlers.
  sigchainlib: hopefully fix darwin build breakage.
  Revert^2: "Make sigchain actually own the signal chain.""