* libguile/validate.h (SCM_VALIDATE_MUTABLE_PAIR):
* libguile/pairs.h (scm_is_mutable_pair): New internal definitions.
* libguile/pairs.c (scm_set_car_x, scm_set_cdr_x): Validate mutable
  pairs.
* libguile/alist.c (scm_assq_set_x, scm_assv_set_x, scm_assoc_set_x):
* libguile/list.c (scm_reverse_x, scm_list_set_x, scm_list_cdr_set_x):
* libguile/srcprop.c (scm_make_srcprops):
* libguile/srfi-1.c (scm_srfi1_append_reverse_x)
  (scm_srfi1_delete_duplicates_x):
* libguile/symbols.c (scm_symbol_fset_x, scm_symbol_pset_x):
* libguile/sort.c (scm_merge_list_x): Use scm_set_car_x / scm_set_cdr_x
  instead of the macros, so as to check for mutable pairs.
  (SCM_VALIDATE_MUTABLE_LIST): New internal helper macro.
  (scm_sort_x, scm_stable_sort_x, scm_sort_list_x): Use
  SCM_VALIDATE_MUTABLE_LIST.
* libguile/vm-engine.c (VM_VALIDATE_MUTABLE_PAIR): New definition.
  (set-car!, set-cdr!): Use VM_VALIDATE_MUTABLE_PAIR.  Fix error message
  for set-cdr!.