Usability: reg fails when you take your time to read the license first...

[bof]

First time user here, with some notes on usability with "acme reg -gen mailto:me@example.com

• first run fails with account key: open /home/me/.config/acme/account.key: no such file or directory

  • solution was to create directory .config/acme by hand

• second run then prompted me to accept the ToS. I took some time to read it. When I came back and entered Y at the prompt I got this error: context deadline exceeded

• a third run fails directly with message 409 urn:acme:error:malformed: Registration key is already in use

• after deleting .config/acme/account.key by hand, a fourth run then worked...

[x1ddos]

Good point, @bof. I'll try to make it better.

[bof]

Fixes in pull request #56