cloud/https: regularize set up of HTTP server
cloud/https: regularize set up of HTTP server

Also fixes a TODO.

Change-Id: Ibbd7935554a5c997fa416310ce60bd75abdde372
Reviewed-on: https://upspin-review.googlesource.com/6992
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

upspin/upspin,upspin/upspin