Rename module 'pipe' to 'spawn-pipe'.

Rename module 'pipe' to 'spawn-pipe'.

* modules/spawn-pipe: New file, renamed from modules/pipe.
(Files, configure.ac, Makefile.am): Update.
(Include): Mention "spawn-pipe.h" instead of "pipe.h".
* modules/pipe: Reduce to an obsolete indirection to 'spawn-pipe'.
* lib/spawn-pipe.h: New file, renamed from lib/pipe.h.
* lib/spawn-pipe.c: New file, renamed from lib/pipe.c. Include
"spawn-pipe.h" instead of "pipe.h".
* m4/spawn-pipe.m4: New file, renamed from m4/pipe.m4. Rename gl_PIPE
to gl_SPAWN_PIPE.
* modules/spawn-pipe-tests: New file, renamed from modules/pipe-tests.
(Files, Makefile.am): Update.
* tests/test-spawn-pipe.sh: New file, renamed from tests/test-pipe.sh.
Update.
* tests/test-spawn-pipe.c: New file, renamed from tests/test-pipe.c.
Include "spawn-pipe.h" instead of "pipe.h".
* lib/csharpcomp.c: Include "spawn-pipe.h" instead of "pipe.h".
* lib/javacomp.c: Likewise.
* lib/javaversion.c: Likewise.
* lib/pipe-filter-gi.c: Likewise.
* lib/pipe-filter-ii.c: Likewise.
* modules/csharpcomp (Depends-on): Add 'spawn-pipe', remove 'pipe'.
* modules/javacomp (Depends-on): Likewise.
* modules/javaversion (Depends-on): Likewise.
* modules/pipe-filter-gi (Depends-on): Likewise.
* modules/pipe-filter-ii (Depends-on): Likewise.
* MODULES.html.sh (Executing programs): Update.
* NEWS: Mention the change.