This does two things:
- Exclude QUIC from go1.16 builds, automatically, for now, since it
doesn't work and just panics.
- Provide some fake listeners and dialers when QUIC is disabled.
These fake listeners and dialers indicate that they are disabled and
unsupported, which silences "Dialing $address: unknown address scheme:
quic" type of stuff which is not super helpful to the user.
The QUIC package is notorious for being incompatible with either too
old or too new Go releases. Currently it doesn't build with Go 1.15 RC
and I want to test the rest with Go 1.15. With this I can do `go run
build.go --tags noquic` to do that.
