watchdog: fix possible segfault during cleanup. On some systems pthread_cancel(...

watchdog: fix possible segfault during cleanup.  On some systems pthread_cancel() caused issues since the watchdog tasks did not restrict themselves to cancel-safe functions.  Resolve this using a simple message pipe so pthread_cancel() is no longer needed during watchdog shutdown.  At the time of writing, the only known distribution to trigger a segfault due to this issue was Slackware 14.0 (gcc 4.7.1, glibc 2.15).  However, the issue does appear to be real so a fix which eliminates the need to use pthread_cancel() is justified.

git-svn-id: svn+ssh://ffado.org/ffado/trunk@2344 2be59082-3212-0410-8809-b0798e1608f0