Implement fmemopen and open_memstream.

Implement fmemopen and open_memstream.
* libc/stdio/fmemopen.c (_fmemopen_r, fmemopen): New file.
* libc/stdio/open_memstream.c (_open_memstream_r, open_memstream):
New file.
* libc/stdio/fopencookie.c (fcwriter): Minor optimization.
* libc/include/stdio.h (dprintf, vdprintf): Group all POSIX 200x
functions together.
(fmemopen, open_memstream): Declare new functions.
* libc/stdio/stdio.tex: Document them.
* libc/stdio/Makefile.am (ELIX_4_SOURCES, CHEWOUT_FILES): Add
fmemopen and open_memstream.
* libc/stdio/Makefile.in: Regenerate.