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.

latin.c

@@ -1352,7 +1352,7 @@ static void gen(int order, random_state *rs, int debug)
     sfree(sq);
 }
 
-void test_soak(int order, random_state *rs)
+static void test_soak(int order, random_state *rs)
 {
     digit *sq;
     int n = 0;
@@ -1375,7 +1375,7 @@ void test_soak(int order, random_state *rs)
     }
 }
 
-void usage_exit(const char *msg)
+static void usage_exit(const char *msg)
 {
     if (msg)
         fprintf(stderr, "%s: %s\n", quis, msg);