FUSE: pass Branches::Ptr by value with empty-branches guards

Previously most FUSE ops took a reference to the live cfg.branches
object. With atomic_load now producing a shared_ptr snapshot, pass
it by value: the op works against a single stable view of the
configuration for its whole lifetime, immune to concurrent
reconfiguration.

Also bail out with -ENOENT when the policy returns an empty branch
set rather than dereferencing branches[0]. This was reachable if a
policy rejected every branch (e.g. everything readonly while
looking for a writable target).

Covers access, chmod, chown, create, getattr, getxattr, ioctl,
link, listxattr, mkdir, mknod, open, readdir (cor/cosr),
readlink, removexattr, rename, rmdir, setxattr, statfs, statx,
symlink, truncate, unlink, utimens.

While here, in _getattr only dereference st_->st_mode when lstat
succeeded (it is otherwise uninitialized).

src/fuse_readlink.cpp

@@ -101,7 +101,7 @@ _readlink_core(const fs::path &basepath_,
 static
 int
 _readlink(const Policy::Search &searchFunc_,
-          const Branches       &ibranches_,
+          const Branches::Ptr   ibranches_,
           const fs::path       &fusepath_,
           char                 *buf_,
           const size_t          bufsize_,