include/ipc: Don't dereference symlinks when subscribing to a topic

include/ipc: Don't dereference symlinks when subscribing to a topic

The `_ipc_endpoint_topic_subscribe()' function dereferences symbolic
links when subscribing to a topic, causing symbolic links to be
created in the wrong directory if the topic already exists.
This commit changes the flags that `_ipc_endpoint_topic_subscribe()'
passes to `ln' so that the target won't be dereferenced if it refers
to a symbolic link.