Convert using rsc.io/c2go rev a97ff47.

Notable changes:
- %% in format string now correctly preserved
- reintroduce "signal handler" to hide internal faults
  after errors have been printed

Change-Id: Ic5a94f1c3a8015a9054e21c8969b52d964a36c45
Reviewed-on: https://go-review.googlesource.com/5633Reviewed-by: Rob Pike <r@golang.org>