net: make lookupPort case-insensitive on Plan 9
net: make lookupPort case-insensitive on Plan 9

The CL 28951 added TestLookupPort_Minimal, which was failing
on Plan 9, because lookupPort was not case-insensitive.

Change-Id: Ic80dd29dad4ffd1c84c2590e3d5d0e588ab2e6c2
Reviewed-on: https://go-review.googlesource.com/29051
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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