net: fix looking up port numbers starting with numbers.

net: fix looking up port numbers starting with numbers.

LookupPort() correctly parses service names beginning with numerals by
implementing a new parser, mainly taken from strconv/atoi.go.

Also testes some previously undefined behaviours around port numbers
larger than 65535 that previously could lead to some tests fail with
EOPNOTSUPP (Operation Not Supported).

Fixes #14322

Change-Id: I1b90dbed434494723e261d84e73fe705e5c0507a
Reviewed-on: https://go-review.googlesource.com/19720
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>