add <optgroup> to #server dropbox to sort different clusters

[DNS-Leo]

I already fiddled with the code, but so far I'm too successful, however; I wish to be able to sort servers in the dropbox overview, using the <optgroup> tag.
People may like to order based on node-location or function (auth/rslv) or whatever.

It will be close to impossible to program matches on hostname, but maybe it's possible to make a default <optgroup> + allow to define custom ones, and have a key/value list of known hosts, which would place them correctly in the generated HTML.