* lib/string-buffer.h: Include string-desc.h.
(sb_append1, sb_append_desc): New declarations.
(sb_append_c): Renamed from sb_append.
(sb_contents, sb_contents_c, sb_dupfree): New declarations.
(sb_dupfree_c): Renamed from sb_dupfree.
* lib/string-buffer.c (sb_append1, sb_append_desc): New functions.
(sb_append_c): Renamed from sb_append.
(sb_contents, sb_contents_c, sb_dupfree): New functions.
(sb_dupfree_c): Renamed from sb_dupfree. Optimize.
* modules/string-buffer (Depends-on): Add string-desc.
* tests/test-string-buffer.c (main): Use sb_append_c instead of
sb_append. Use sb_dupfree_c instead of sb_dupfree. Test also sb_append1,
sb_append_desc, sb_contents.
* NEWS: Mention the changes.
* lib/string-buffer.h (sb_appendvf, sb_appendf): Document that errno is
set upon failure.
* lib/string-buffer-printf.c: Include <errno.h>.
(sb_appendvf): Call vsnzprintf instead of vsnprintf. Ensure errno is
set upon failure.
(sb_appendf): Likewise.
* modules/string-buffer (Depends-on): Add vsnzprintf-posix. Remove
vsnprintf-posix.
* tests/test-string-buffer.c (main): Test error code from sb_appendf.
* lib/string-buffer-printf.c: New file, extracted from
lib/string-buffer.c.
* lib/string-buffer.c: Don't include <stdarg.h>.
(sb_ensure_more_bytes): Declare. Make non-static.
(sb_appendvf, sb_appendf): Moved to lib/string-buffer-printf.c.
* modules/string-buffer (Files): Add lib/string-buffer-printf.c.
(Makefile.am): Arrange to compile string-buffer-printf.c.
* lib/string-buffer.h: New file.
* lib/string-buffer.c: New file.
* modules/string-buffer: New file.
* doc/posix-functions/open_memstream.texi: Mention the new module.
