When a child did not come up within cli_timeout, varnishd startup
would hang indefinitely.

We add startup_timeout specifically for child startup.

To facilitate the transition, we use the maximum of cli_timeout and
start_timeout (suggested by Dridi, thank you) and add a tip if
startup_timeout is not used.

We avoid the previous harsh exit(1), primarily to make the vtc_varnish
facility work.

The test case uses both vtc_varnish and vtc_process to exercise the
different code paths for implicit startup vs. cli "start".

Fixes #3940