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When a fatal signal is received, we now write a message to the log
that looks like this:

  F/libc    ( 1540): Fatal signal 11 (SIGSEGV) at 0xdeadd00d (code=1)

This is useful for debugging fatal signals that turn out not to be
fatal.  This also changes the signal reset from SIG_IGN to SIG_DFL,
so that future non-fatal fatal signals are fatal.

The code that blocked SIGUSR1 to avoid being interrupted by the GC
has been removed.

Also, fix minor issues in format_buffer().

Bug 5035703

Change-Id: I8940af47297b5dcf3cf33537e3483ca5334ed565