eventscript: remove cb_status, fix uninitialized bug when monitoring aborted

eventscript: remove cb_status, fix uninitialized bug when monitoring aborted

(Reapplied with merge after accidental revert)

Previously we updated cb_status a each script finished.  Since we're storing
the status anyway, we can calculate it by iterating the scripts array
itself, providing clear and uniform behavior on all code paths.

In particular, this fixes a longstanding bug when we abort monitor
scripts to run some other script: the cb_status was uninitialized.  In
this case, we need to hand *something* to the callback; 0 might make
us go healthy when we shouldn't.  So we use the last status (normally,
this will be the just-saved current status).

In addition, we make the case of failing the first fork for the script
and failing other script forks the same: the error is returned via the
callback and saved for viewing through 'ctdb scriptstatus'.

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