Fix bug #11519 with relocation of buffer text during regex search.

Fix bug #11519 with relocation of buffer text during regex search.

 src/lisp.h [REL_ALLOC]: Add prototypes for external functions
 defined on ralloc.c.
 src/buffer.c [REL_ALLOC]: Remove prototypes of
 r_alloc_reset_variable, r_alloc, r_re_alloc, and r_alloc_free,
 they are now on lisp.h.
 src/ralloc.c (r_alloc_inhibit_buffer_relocation): New function.
 src/search.c (search_buffer): Use it to inhibit relocation of buffer
 text while re_search_2 is doing its job, because re_search_2 is
 passed C pointers to buffer text.