uchar: Include the necessary snippets. * lib/uchar.in.h: Add insertion points for _GL_ARG_NONNULL and _GL_WARN_ON_USE. * modules/uchar (Depends-on): Add snippet/arg-nonnull, snippet/warn-on-use. (Makefile.am): In uchar.h, include the ARG_NONNULL_H and WARN_ON_USE_H snippets.