Added in compatibility for NFSv4

[uriel1998]

• Version 1.1 and below used an RPC check via the showmount command to check available remote host mountpoints, which works great with NFSv2 and NFSv3. This does NOT work with NFSv4 (see this article), so if an RPC error is detected, it now presumes that NFSv4 is being used and attempts the mount anyway if the server exists. If mounting does not occur (e.g. "backgrounding" due to a misconfiguration) it immediately unmounts the mountpoint instead of leaving it in the "backgrounded" state. No configuration changes are needed.