This is useful, especially in testing, since your test
hosts might not reliabliy start the server within two
seconds, so we do a definite check before progressing.
By the same token, after `kill $server_pid` wait for
the pid to be gone from the process list.
Since now the script can end prematurely, I also added
a cleanup function to make sure the temporary certs are
removed in any case.
n.b. this could also be done with the use of `trap 'fn'
ERR` but that felt like a bit too much magic for this
short a script.
Jan Lehnardt