When we receive a response that contains at least one packet, permit 2
immediate polls. Polls are also timeout-driven, using an expoentially
increasing delay as before.

This adds --quic-tls-cert and --quic-tls-key options to the server
(separate from --cert and --key); and a quic-tls-pubkey SOCKS arg (with
equivalent --quic-tls-pubkey command-line option) to the client.

This package defines how we encapsulate discrete discrete datagrams into
an HTTP request or response body, with optional padding.

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.

This doesn't hurt, but we're still reliant on Firefox for using the
global "browser" object (instead of "chrome"), and especially for the
proxy code, which has a totally different API in Chrome.

A typo during debugging caused the callback to crash and have no effect.

The WebExtension needs a JSON "host manifest" that both authorizes the
extension to run a native executable, and tells the browser where to
find the native executable. The path inside the manifest needs to be an
absolute path, so we cannot just plunk down a static file; we have to
know the path to where the browser is installed. meek-client-torbrowser
rewrites the manifest on each startup, where the browser expects to find
it.

The is mostly self-contained and compatible with previous behavior, with
one small exception on windows. On mac and linux, the browser expects to
find the manifest in a well-known location (relative to $HOME, which in
our case is inside the browser's directory tree or the ancillary
TorBrowser-Data directory). But on windows, the path to the manifest
needs to be stored in the registry. So meek-client-torbrowser not only
writes the manifest file, it also writes a registry key pointing to the
file. I'd like to try and find a way to do this that doesn't require
modifying global state like this.

This patch is tested on linux and windows but not mac.

This is what enabled me to find the proxyDNS problem fixed by the
previous commit. Without it, there was no error message in the browser
console.

This is to work around a Firefox patch that otherwise prevents DNS
requests from being made when proxyType=="direct".

Compare to
https://gitweb.torproject.org/pluggable-transports/meek.git/commit/?id=32ca07ca008f5d09492f4d2782130bd54c52acf5