eventscript: don't do debugging system() from inside signal handler

eventscript: don't do debugging system() from inside signal handler

In the case of a timeout, we dump a log of what's happening to a file
in /tmp.  We do it from the signal handler, which is an unreliable hack
(BZ58365).

Instead, create another (lower-priority) child to do the dump, then
kill the timedout script.

Note that this doesn't quite work as intended (the dump is often run
after the script has been killed), so the next patch resolves this.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>