debuggerd can't handle debuggerd crashes

When a process receives a fatal signal, it connects to debuggerd
and initiates a conversation over a socket.  When the crashing
process is debuggerd, the conversation stalls forever, and you
stop getting native crash details.

This resets the signal handlers to SIG_DFL, so that debuggerd
simply crashes and gets restarted.

Bug 5035703

Change-Id: Ie081d710d046f701bca7e7d8ea3e1d24766d9f61