Track all child process so we never send a signal to an unrelated process (our child...

Track all child process so we never send a signal to an unrelated process (our child died  and kernel wrapped the pid-space and reused the pid for a different process

Wrap all creation of child processes inside ctdb_fork() which is used to track all processes we have spawned.
Capture SIGCHLD to track also which child processes have terminated.

Wrap kill() inside ctdb_kill() and make sure that we never send a !0 signal to a child process pid that has already terminated (and might have been replaced with a

(This used to be ctdb commit f73a4b1495830bcdd094a93732a89dd53b3c2f78)