RPC: HTTP server uses its own ReadHTTPRequestLine() rather than reusing ReadHTTPStatus() from the client mode.

The following additional HTTP request validations are added, both in line with
existing HTTP client practice:

1. HTTP method must be GET or POST. Most clients use POST, some use GET. Either way, this continues to work.
2. HTTP URI must start with "/" character. Normal URI is "/" (a 1-char string), so this is fine.