Substantial reworking of Solo so that it implements both Sudoku-X

(require both main diagonals to have one of every digit in addition
to all the usual constraints) and Jigsaw Sudoku (replace the array
of rectangular sub-blocks with the sub-blocks being random
polyominoes). To implement the latter, I've moved my `divvy.c'
library routine out of the `unfinished' subdirectory.

Jigsaw mode is currently an undocumented feature: you enable it by
setting the rows parameter to 1 (and the columns parameter to your
desired grid size, which unlike normal Sudoku can be anything you
like including a prime number). The reason it's undocumented is
because generation times are not yet reliably short: sometimes
generating a jigsaw-type puzzle can hang for hours and still get
nowhere. (The algorithm should terminate in principle, but not in
any time you're prepared to wait.) I _think_ I know how to solve
this, but have yet to try it. Until then, jigsaw mode will remain a
hidden feature.

Printing of X-type puzzles is also substandard at present, because
the current print-colour API replaces the desired light shading of
the X-cells with heavy diagonal hatching. I plan to adjust the API
imminently to address this.

[originally from svn r7974]

puzzles.h

@@ -381,6 +381,12 @@ void reset_combi(combi_ctx *combi);
 combi_ctx *next_combi(combi_ctx *combi); /* returns NULL for end */
 void free_combi(combi_ctx *combi);
 
+/*
+ * divvy.c
+ */
+/* divides w*h rectangle into pieces of size k. Returns w*h dsf. */
+int *divvy_rectangle(int w, int h, int k, random_state *rs);
+
 /*
  * Data structure containing the function calls and data specific
  * to a particular game. This is enclosed in a data structure so