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_removexattr.cpp

@@ -63,7 +63,7 @@ static
 int
 _removexattr(const Policy::Action &actionFunc_,
              const Policy::Search &searchFunc_,
-             const Branches       &ibranches_,
+             const Branches::Ptr   ibranches_,
              const fs::path       &fusepath_,
              const char           *attrname_)
 {
@@ -74,6 +74,8 @@ _removexattr(const Policy::Action &actionFunc_,
   rv = actionFunc_(ibranches_,fusepath_,obranches);
   if(rv < 0)
     return rv;
+  if(obranches.empty())
+    return -ENOENT;
 
   ::_removexattr_loop(obranches,fusepath_,attrname_,&prv);
   if(prv.errors.empty())
@@ -85,6 +87,8 @@ _removexattr(const Policy::Action &actionFunc_,
   rv = searchFunc_(ibranches_,fusepath_,obranches);
   if(rv < 0)
     return rv;
+  if(obranches.empty())
+    return -ENOENT;
 
   return prv.get_error(obranches[0]->path);
 }