net.lisp: net.lisp: Refactor `string-subipnet' and its friends.

  * Kill `ipnet-subnet', which wasn't used anywhere else anyway.
    Replace with `check-subipnet', with a different interface.

  * Add `:slashp' argument to `parse-subnet', controlling whether it
    bothers to detect a `/'.

  * Introduce `parse-subipnet' as a higher-level interface to
    `parse-subnet'.