Selfo: no colour starts one move away from winning

[fjvico]

The random start already rejected positions where the first player could win
with a single move. This extends the check to both colours.

Why: with the pie rule the second player can take the first player's colour,
and the second player could also win at once on the small board. In a
simulation of the generator (about 2000 starts per size), the radius-2 board
let through ~2% of starts where the second colour was one move from winning.
The larger boards had none.

• New canWinInOne helper; randomStart rejects a position if either colour
  can win in one move.
• Tests: no start has a colour one move from winning (radius 2, radius 2 with
  no-enclosure, and the default board), and a unit test for the helper.
• gameinfo.urls now links the Wikipedia article on Selfo.
• No translated strings are changed, so no locale files are touched.

npm run build passes and all Selfo tests pass.