1.0.26.8: QSHOW changes, bug reporting guidelines

1.0.26.8: QSHOW changes, bug reporting guidelines

- change runtime.h so that a simple '#define QSHOW_SIGNAL 1' turns
QSHOW automatically and defaults to blocking signals during printing

- add notes to BUGS on how to report bugs related to signal handling

- kill a warning in thread.c in code conditional on QSHOW_SIGNAL

- add #include <stdio.h> to x86{-64,}-darwin-os.c so that it compiles
  with QSHOW

- add comment explaining the previous commit