acme: improve http-01 challenge API

This makes http-01 handling more consistent with tls-sni-* methods
and, hopefully, future implementation such as dns-01 and oob-01.

Change-Id: If1a4a47ee8528ef19960fee4e6860392c1813fe6
Reviewed-on: https://go-review.googlesource.com/27011
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed