cloud/https: Convert from rsc.io/letsencrypt to x/crypto/acme/autocert.
cloud/https: Convert from rsc.io/letsencrypt to x/crypto/acme/autocert.

Fixes #71.

Change-Id: Ied0574d95b7fc18c8475bf2815e0f2b9491bc595
Reviewed-on: https://upspin-review.googlesource.com/6943
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin