Rename module 'c++defs' to 'snippet/c++defs'.

Rename module 'c++defs' to 'snippet/c++defs'.

* modules/snippet/c++defs: Renamed from modules/c++defs.
(Files, Makefile.am): Update.
* build-aux/snippet/c++defs.h: Renamed from build-aux/c++defs.h.
* modules/arpa_inet (Depends-on): Update.
* modules/ctype (Depends-on): Update.
* modules/dirent (Depends-on): Update.
* modules/fcntl-h (Depends-on): Update.
* modules/glob (Depends-on): Update.
* modules/iconv-h (Depends-on): Update.
* modules/langinfo (Depends-on): Update.
* modules/locale (Depends-on): Update.
* modules/math (Depends-on): Update.
* modules/netdb (Depends-on): Update.
* modules/poll-h (Depends-on): Update.
* modules/pty (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_ioctl (Depends-on): Update.
* modules/sys_select (Depends-on): Update.
* modules/sys_socket (Depends-on): Update.
* modules/sys_stat (Depends-on): Update.
* modules/sys_time (Depends-on): Update.
* modules/sys_wait (Depends-on): Update.
* modules/termios (Depends-on): Update.
* modules/time (Depends-on): Update.
* modules/unistd (Depends-on): Update.
* modules/wchar (Depends-on): Update.
* modules/wctype-h (Depends-on): Update.