* src/lisp.h: Add new Lisp_Misc_User_Ptr type.
(XUSER_PTR): New User_Ptr accessor.
* src/alloc.c (make_user_ptr): New function.
(mark_object, sweep_misc): Handle Lisp_Misc_User_Ptr.
* src/data.c (Ftype_of): Return 'user-ptr' for user pointer.
(Fuser-ptrp): New user pointer type predicate function.
(syms_of_data): New 'user-ptrp', 'user-ptr' symbol. New 'user-ptrp'
subr.
* src/print.c (print_object): Add printer for User_Ptr type.