Check state is valid at the end of a move in Pearl

A Pearl move string contains a sequence of sub-moves, each of which
can affect the state of the connection between the centre of a square
and one of its edges.  interpret_move() generates these in pairs so
that the two halves of a connection between the centres of adjacent
squares stay in the same state.

If, however, a save file contains mismatched half-moves,
execute_move() should ideally return NULL rather than causing an
assertion failure.  This has to be checked at the end of the whole
move string, so I've arranged for check_completion() to return a
boolean indicating whether the current state (and hence the move
preceding it) is valid.  It now returns 'false' when a connection
stops at a square boundary or when it goes off the board.  These
conditions used to be assertion failures, and now they just cause the
move to be rejected.

This supersedes the check for off-board connections added in 15f4fa8,
since now check_completion() can check for off-board links for the
whole board at once.

This save file trivially demonstrates the problem, causing
"dsf_update_completion: Assertion `state->lines[bc] & F(dir)' failed"
without this fix:

SAVEFILE:41:Simon Tatham's Portable Puzzle Collection
GAME    :5:Pearl
PARAMS  :5:6x6t0
CPARAMS :5:6x6t0
DESC    :17:BbBfWceBbWaBWWgWB
NSTATES :1:2
STATEPOS:1:2
MOVE    :6:R1,0,0

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