ipn/ipnlocal: add HTTP/2 h2c server support to peerapi on non-mobile platforms

To make ExitDNS cheaper.

Might not finish client-side support in December before 1.20, but at
least server support can start rolling out ahead of clients being
ready for it.

Tested with curl against peerapi.

Updates #1713

Change-Id: I676fed5fb1aef67e78c542a3bc93bddd04dd11fe
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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