eventscript: fork() a child for each script.

eventscript: fork() a child for each script.

We rename child_run_scripts() to child_run_script(), because it now
runs a single script rather than walking the list.  When it's
finished, we fork the next child from the ctdb_event_script_handler()
callback.

ctdb_control_event_script_init() and ctdb_control_event_script_finished()
are now called directly by the parent process; the child still calls
ctdb_ctrl_event_script_start() and ctdb_ctrl_event_script_stop() before
and after the script.

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