ipn/ipnlocal: also accept service IP IPv6 literal in brackets for quad100

The fix in 4fc8538e2 was sufficient for IPv6. Browsers (can?) send the
IPv6 literal, even without a port number, in brackets.

Updates tailscale/corp#7948

Change-Id: I0e429d3de4df8429152c12f251ab140b0c8f6b77
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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