Reduce suicidal tendencies

Some changes to debuggerd:

(1) Send a SIGSTOP to the target process before doing any of the
PTRACE_ATTACH work.  This causes the threads that weren't hit by
the signal to stop moving, which seems to get rid of the "ptrace
detach failed: No such process" problems that result in missed
stack traces and debuggerd self-immolation.

(2) SIGPIPE was in the list of "interesting" signals caught by
debugger_signal_handler(), but debuggerd didn't recognize it, and
you'd get "unexpected signal 13" complaints.  It's now in the list.

(3) Fixed a bit-rotted XLOG.

Change-Id: I33297ea6b09ce450671c07dc25846f684afbf480