Debugging facility to check whether 'const char *' points to

Debugging facility to check whether 'const char *' points to
relocatable data of non-pure Lisp string.
* alloc.c (maybe_lisp_pointer): New function, refactored out of ...
(mark_maybe_pointer): ... adjusted user.
(relocatable_string_data_p): New function.
* lisp.h (relocatable_string_data_p): Add prototype.
* xdisp.c (message_with_string): If ENABLE_CHECKING, make sure
the pointer to relocatable Lisp data is not used.