ipsets: add support for specifying entries

Introduce a new list option "entry" which can be used to specify entries
to add to the ipset, e.g.

    config ipset
      option name test
      ...
      list entry 1.2.3.4,8080
      list entry 5.6.7.8,8081

Also introduce a new option "loadfile" which refers to an external file
containing set entries to add, with one item per line.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>