Fix the range of `random' on 64-bit platforms

Fix the range of `random' on 64-bit platforms

For > 32 bit integers still in the fixnum range, scm_random() would
return random numbers with a lower range than specified.

* libguile/random.c (scm_i_mask32): New static inline function.
  (scm_c_random): Use `scm_i_mask32'.
  (scm_c_random64): New function, 64-bit variant of scm_c_random.
  (scm_random): Use `scm_c_random64' instead of forming the 64-bit random
  number in a bogus way.
* libguile/random.h: Added `scm_c_random64'.