Cleanup up non-prototyped functions (static) and const-correctness

More fixes based on the compiler directives -Wcast-qual -Wwrite-strings
-Wmissing-prototypes -Wmissing-declarations. Not everything with regards
to -Wcast-qual has been fixed as some have unwanted consequences for the
rest of the code.

library/ecp.c

@@ -1296,7 +1296,7 @@ int ecp_self_test( int verbose )
     ecp_point R;
     mpi m;
     unsigned long add_c_prev, dbl_c_prev;
-    char *exponents[] =
+    const char *exponents[] =
     {
         "000000000000000000000000000000000000000000000000", /* zero */
         "000000000000000000000000000000000000000000000001", /* one */