Related: tpo/anti-censorship/team/-/issues/86

Updated a test relating to the server_name extension when an IP address
is dialed.
https://github.com/refraction-networking/utls/pull/100

sync.Once does not prevent other goroutines from accessing a variable
that has not been defined yet.

We need to guard against concurrent modification of rt.rt, but once it
is set, we many concurrently call rt.rt.RoundTrip. The way this was
written before, it was preventing more than one RoundTrip from happening
at once. (Which was not noticeable, because the protocol serialized all
RoundTrips.)

To match documentation added in
7aa47e7f.

The crypto/tls server, since go1.12, requires that if a server_name
extension is present, it is not empty. With go1.12 and later, this test
was failing with
	proxy_test.go:95: local error: tls: unexpected message

The specific commit that added the empty server_name check was:
https://go-review.googlesource.com/c/go/+/144115
https://github.com/golang/go/commit/4c8b09e9183390d6ab80d3f53a9fe5f6ace92f06#diff-f2f5faeac5da94dc7735456f798177b53421f827b8bfc242b56f116b8e5c7e62R264
Note the comment "enforcing a couple more 'must not be empty' on the
unmarshaling side".

Wireshark also complains in a packet capture:
	[Expert Info (Warning/Protocol): Vector length 0 is smaller than minimum 1]

https://bugs.torproject.org/tpo/anti-censorship/pluggable-transports/meek/40002

This ought to have been removed in
bc887de6.

https://bugs.torproject.org/tpo/anti-censorship/pluggable-transports/meek/40002

github.com/refraction-networking/utls

Adds
	HelloFirefox_65
	HelloChrome_72
	HelloIOS_12_1

Clone is new in go1.13.
https://github.com/golang/go/issues/26013

https://bugs.torproject.org/29611#comment:6

https://bugs.torproject.org/29611

app.update.enabled doesn't exist anymore:
https://bugzilla.mozilla.org/show_bug.cgi?id=1420514

This is copied from
projects/tor-browser/Bundle-Data/PTConfigs/meek-http-helper-user.js in
tor-browser-build.git commit 4792ed5246d389caad036290e9ed34ff0ae1f0ad.

The difference here is that we don't send a JSON error response as a
result of errors at this stage.

The native component actually doesn't need to introspect the objects it
passes to the browser, except for the "error" property of a response
when it wants to report its own error.

Re-use the catch handler.