Add range check on API_PORT that is good for the implied UDP_PORT also.
legal ports in the OS are 0 to 65535
I had hit a ugly stack trace when I gave a bad port, or (for an separate reason),
h2o cycled thru all ports and couldn't find a good one for the ip I provided.
Avoids the ugly stack trace on bad port to other methods, with a better message to user.
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