http/ftp: Tweak to avoid depending on libc's NSS.

* guix/build/http.scm (open-connection-for-uri): New procedure.
  (http-fetch): Use it.  Pass the result as a #:port argument to
  `http-get'.
  Add hack to modify the `set-port-encoding!' binding in (web response).

* guix/ftp-client.scm (ftp-open): Add optional `port' parameter,
  defaulting to 21.  When calling `getaddrinfo', convert PORT to a
  string and pass AI_NUMERICSERV when PORT is a number.