Freeciv

We already have the logic in the server to enumerate .serv files, to support PACKET_RULESET_CHOICES for single-player ruleset selection. We may as well make it easy to see what rulesets there are at the server prompt; we have a lot these days.

(This does further entrench the convention that a .serv file on the data path is associated with a ruleset, where strictly they could be unrelated to rulesets.)

• There doesn't seem a good reason to cache the list, as we do currently. (The caching has been there since the feature was first added.)
• Handle overlong / too many rulesets better (don't send client truncated filenames in PACKET_RULESET_CHOICES; add logging).