Use `Manager.HTTPHandler` (ACME "HTTP-01" challenge) for automatic certificates

Let's Encrypt disabled the TLS-SNI challenge, which is the basis of the autocert package that meek-server uses for automatic TLS certificates:

• tls-sni challenge disabled I've informed the public meek-server operators about this and asked that they be ready with manual certificates in the short term.

The autocert package recently added support for the HTTP-01 challenge. It requires the server to listen on port 80.

Further reading:

• 2018.01.09 Issue with TLS-SNI-01 and Shared Hosting Infrastructure
• 2018.01.11 Update Regarding ACME TLS-SNI and Shared Hosting Infrastructure
• TLS-SNI challenges disabled for most new issuance
• https://twitter.com/bradfitz/status/951909513593958400

  Use the #golang autocert package? You need to update your code due to @LetsEncrypt changes. You need to use this now: https://godoc.org/golang.org/x/crypto/acme/autocert#Manager.HTTPHandler See the example: https://godoc.org/golang.org/x/crypto/acme/autocert#example-Manager Everybody's sorry. Tears all around. 😢

• x/crypto/acme/autocert: Support http-01 challenge (GitHub #21890)
• acme/autocert: support http-01 challenge type