Write access to pure space in dumped executable

[YAMAMOTO Mitsuharu]

I found a few types of write access to the pure space in the normal
execution of the dumped one:

1. pure_cons calls from init_process (process.c).

2. XSETCAR/XSETCDR in the CHECK_CHARACTER_CAR/CDR macros (character.h).

   What are these XSETCAR/XSETCDR for?

3. write access to empty_unibyte/multibyte_string (lisp.h, alloc.c)

   If the length of the string to be allocated is 0,
   make_uninit_string (or make_uninit_multibyte_string) returns
   empty_unibyte_string (or empty_multibyte_string, respectively),
   which is allocated in the pure space.  So,

   3.1. STRING_SET_UNIBYTE tries to write the `size_byte member' of
        struct Lisp_String when the arg STR is actually
        empty_unibyte_string.
   
   3.2. `*p = 0;' around the end of Fmake_string tries to write the
        string data of these empty strings if the arg LENGTH is 0.

                                     YAMAMOTO Mitsuharu
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