Rename module 'arg-nonnull' to 'snippet/arg-nonnull'.

Rename module 'arg-nonnull' to 'snippet/arg-nonnull'.

* modules/snippet/arg-nonnull: Renamed from modules/arg-nonnull.
(Files, Makefile.am): Update.
* build-aux/snippet/arg-nonnull.h: Renamed from build-aux/arg-nonnull.h.
* modules/argv-iter (Depends-on): Update.
* modules/arpa_inet (Depends-on): Update.
* modules/dirent (Depends-on): Update.
* modules/fcntl-h (Depends-on): Update.
* modules/fnmatch (Depends-on): Update.
* modules/getopt-posix (Depends-on): Update.
* modules/glob (Depends-on): Update.
* modules/iconv-h (Depends-on): Update.
* modules/inttypes-incomplete (Depends-on): Update.
* modules/locale (Depends-on): Update.
* modules/math (Depends-on): Update.
* modules/netdb (Depends-on): Update.
* modules/search (Depends-on): Update.
* modules/signal (Depends-on): Update.
* modules/spawn (Depends-on): Update.
* modules/stdio (Depends-on): Update.
* modules/stdlib (Depends-on): Update.
* modules/string (Depends-on): Update.
* modules/strings (Depends-on): Update.
* modules/sys_socket (Depends-on): Update.
* modules/sys_stat (Depends-on): Update.
* modules/sys_time (Depends-on): Update.
* modules/sys_times (Depends-on): Update.
* modules/sys_utsname (Depends-on): Update.
* modules/time (Depends-on): Update.
* modules/unistd (Depends-on): Update.
* modules/wchar (Depends-on): Update.
* MODULES.html.sh (Support for building libraries and executables):
Update.