Don't keep "disabled" substituters running

For instance, it's pointless to keep copy-from-other-stores running if
there are no other stores, or download-using-manifests if there are no
manifests.  This also speeds things up because we don't send queries
to those substituters.

src/libutil/util.cc

@@ -790,6 +790,7 @@ void Pid::kill()
 
 int Pid::wait(bool block)
 {
+    assert(pid != -1);
     while (1) {
         int status;
         int res = waitpid(pid, &status, block ? 0 : WNOHANG);