util-lib: use trailing slash in chase_symlinks, fd_is_mount_point, path_is_mount_point

util-lib: use trailing slash in chase_symlinks, fd_is_mount_point, path_is_mount_point

The kernel will reply with -ENOTDIR when we try to access a non-directory under
a name which ends with a slash. But our functions would strip the trailing slash
under various circumstances. Keep the trailing slash, so that

path_is_mount_point("/path/to/file/") return -ENOTDIR when /path/to/file/ is a file.

Tests are added for this change in behaviour.

Also, when called with a trailing slash, path_is_mount_point() would get
"" from basename(), and call name_to_handle_at(3, "", ...), and always
return -ENOENT. Now it'll return -ENOTDIR if the mount point is a file, and
true if it is a directory and a mount point.

v2:
- use strip_trailing_chars()

v3:
- instead of stripping trailing chars(), do the opposite — preserve them.