Fix some bugs when close_to and find were asked to list stations

Don't crash when trying to show unknown station distance.

We were trying to sort a Dist against a float.

Don't crash in find_stations() when there are many systems.

We can only build a parameterised query with at most
SQLITE_MAX_VARIABLE_NUMBER parameters. By default that is 999, although
there is no easy way to determine programatically what the
implementation limit is, so we assume it is 999 and split the query into
batches of at most that size.