* 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.