Move allocation with mmap here, from ralloc.c. Change

Move allocation with mmap here, from ralloc.c.  Change
conditional compilation on REL_ALLOC_MMAP to USE_MMAP_FOR_BUFFERS.
(mmap_alloc, mmap_free, mmap_realloc) [REL_ALLOC_MMAP]: Renamed
from former r_alloc_* functions in ralloc.c.
(mmap_page_size, mmap_initialized_p) [REL_ALLOC_MMAP]: New
variables.
(MEM_ALIGN) [REL_ALLOC_MMAP]: New macro.
(mmap_init) [REL_ALLOC_MMAP]: New function.
(alloc_buffer_text, enlarge_buffer_text, free_buffer_text): New
functions replacing macros BUFFER_ALLOC, BUFFER_REALLOC, and
BUFFER_FREE.