sigchain: allow use of sigprocmask inside handlers.

When we're inside a signal handler, we don't need to filter out claimed
signals from sigprocmask calls, since return from the signal handler
will restore the previous signal mask. Use the handling_signal thread
local to pass sigprocmask through when we're in a signal handler.

Bug: http://b/36205469
Bug: http://b/36262089
Change-Id: I7943e7a68c7929d9244cef83ddeb3d64243d8840