Add wget(1)-friendly crypted key dump via GET /wallet-ckeys.json

See individual commits.  This provides a slightly different, not-JSON-RPC API endpoint, which makes this HTTP request usable via "wget" and similar utilities.

The content returned and format thereof is certainly open to discussion.

My main goals were
* fix HTTP request line parsing bugs
* tighten HTTP request line validation, including "/" URI (our default JSON-RPC endpoint)
* permit dump of crypted keys, without decrypting them
* demonstrate a not-JSON-RPC API download endpoint