sigchain: make SIG_DFL handling less broken.

Abort with a message instead of reraising the signal, which will just
reenter the signal handler when we return.

Before the libsigchain rewrite, we would try to call SIG_IGN (i.e. 1)
as a function, with no error reporting, this improves it to an error
message.

Bug: http://b/32631264
Bug: http://b/36262089
Test: none
Change-Id: Ie0ba58b301dea48eaf3223a83883cd4af3d1576f