Delegate SIG_DFL on sigaction to libc.

  In the case when user registers SIG_DFL hanlder
  sigchainlib's handler tends to go into infinite loop
  because the function handling signals resets signal
  using by calling to signchain's own implementation.

  This cl fixes the problem by passing sigcations with
  SIG_DFL to the next sigaction (usually libc's).

Bug: 19594886
Change-Id: I9eecf9afd1c7e6d1fe3cd1d4fc506383ecbebe04