Add obstack-printf and obstack-printf-posix modules.

Add obstack-printf and obstack-printf-posix modules.

* modules/obstack-printf: New file.
* modules/obstack-printf-posix: Likewise.
* MODULES.html.sh (Misc): Mention them.
* doc/glibc-functions/obstack_printf.texi (obstack_printf):
Likewise.
* doc/glibc-functions/obstack_vprintf.texi (obstack_vprintf):
Likewise.
* modules/stdio (Makefile.am): Accomodate new modules.
* m4/stdio_h.m4 (gl_STDIO_H_DEFAULTS): Likewise.
* lib/stdio.in.h (rpl_obstack_printf, rpl_obstack_vprintf):
Declare.
* lib/obstack_printf.c (obstack_printf, obstack_vprintf): New
functions.
* m4/obstack-printf.m4 (gl_OBSTACK_PRINTF)
(gl_REPLACE_OBSTACK_PRINTF): New macros
* m4/obstack-printf-posix.m4 (gl_OBSTACK_PRINTF_POSIX): Likewise.
* tests/test-obstack-printf.c: New file.
* modules/obstack-printf-tests: Likewise.
* modules/obstack-printf-posix-tests: Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>