Assorted char * -> const char * API changes.

I went through all the char * parameters and return values I could see
in puzzles.h by eye and spotted ones that surely ought to have been
const all along.

unfinished/path.c

@@ -770,7 +770,7 @@ int main(void)
 #ifdef TEST_GENERAL
 #include <stdarg.h>
 
-void fatal(char *fmt, ...)
+void fatal(const char *fmt, ...)
 {
     va_list ap;