some system-specific adjustments for RNG routines

move some system-specific seed-related stuff from hacklib.c to
a system-specific source file and #define SYS_RANDOM_SEED to
utilize it during build.

Windows changes for random seed generation using
crypto next gen (CNG) api routines.

Corresponding vms changes due to disentangling of VMS and
unix when the unix seed bits got moved (untested).

src/rnd.c

@@ -27,6 +27,7 @@ RND(int x)
 {
     return (isaac64_next_uint64(&rng_state) % x);
 }
+
 #else
 /* "Rand()"s definition is determined by [OS]conf.h */
 #if defined(LINT) && defined(UNIX) /* rand() is long... */