Avoid signed overflow and use size_t in bytevectors.c.

Avoid signed overflow and use size_t in bytevectors.c.

* libguile/bytevectors.c (ULONG_MAX): Remove (unportable) fallback
  definition.  It is no longer used.
  (scm_make_bytevector): Use 'size_t' instead of 'unsigned' for
  indices.  Use 'scm_t_uint8' instead of 'signed char' for elements
  and c_fill.  Previously relied on GNU-C-specific behavior when
  casting integers in the range 128-255 to signed char.
  (GENERIC_INTEGER_ACCESSOR_PROLOGUE, INTEGER_LIST_TO_BYTEVECTOR)
  (scm_bytevector_eq_p, scm_bytevector_copy_x, scm_bytevector_copy)
  (scm_bytevector_to_u8_list, scm_u8_list_to_bytevector)
  (scm_bytevector_fill_x): Use 'size_t' instead of 'unsigned' for
  indices.  Use 'scm_t_uint8' for elements.