canonicalize, canonicalize-lgpl: honor // if distinct from /

canonicalize, canonicalize-lgpl: honor // if distinct from /

* modules/canonicalize (Files): Add double-slash-root.m4.
* modules/canonicalize-lgpl (Files): Likewise.
* m4/canonicalize.m4 (gl_FUNC_CANONICALIZE_FILENAME_MODE)
(gl_CANONICALIZE_LGPL_SEPARATE): Add dependency.
* lib/canonicalize.c (DOUBLE_SLASH_IS_DISTINCT_ROOT): Provide
fallback definition.
(canonicalize_filename_mode): Use it to protect //.
* lib/canonicalize-lgpl.c (DOUBLE_SLASH_IS_DISTINCT_ROOT)
(__realpath): Likewise.
* tests/test-canonicalize.c (main): Test this.
* tests/test-canonicalize-lgpl.c (main): Likewise.
* modules/canonicalize-tests (Depends-on): Add same-inode.
* modules/canonicalize-lgpl-tests (Depends-on): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>