Fix sigchainlib's implementation of sigaction
Fix sigchainlib's implementation of sigaction

  Correctly handles the case when old_action == new_action

Bug: 18740478
Change-Id: I97092318439e4f6f0a2513d4336496c72f8c5599

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst