Mark many more function (and some objects) static

I noticed commit db3b531e2cab765a00475054d2e9046c9d0437d3 in the history
where Simon added a bunch of "static" qualifiers.  That suggested that
consistently marking internal functions "static" is desirable, so I
tried a build using GCC's -Wmissing-declarations, which requires prior
declaration (presumed to be in a header file) of all global functions.

This commit makes the GTK build clean under GCC's
-Wmissing-declarations.  I've also adding "static" to a few obviously
internal objects, but GCC doesn't complain about those so I certainly
haven't got them all.

magnets.c

@@ -2492,11 +2492,11 @@ const struct game thegame = {
 const char *quis = NULL;
 bool csv = false;
 
-void usage(FILE *out) {
+static void usage(FILE *out) {
     fprintf(out, "usage: %s [-v] [--print] <params>|<game id>\n", quis);
 }
 
-void doprint(game_state *state)
+static void doprint(game_state *state)
 {
     char *fmt = game_text_format(state);
     printf("%s", fmt);