nfs (v3): Allow listing of virtual directories even if they cannot be mounted
When attempting to mount the server's exported filesystems in NFSProtocolV3::openConnection(), record the export path even if the mount failed. This allows them to be listed as virtual directories, even though any other access needs a file handle. Detect that situation in NFSProtocol::getFileHandle() and return an error. Detect and warn about a mount which may have failed due to reserved port restrictions. Correct the virtual directory path prefix check in NFSProtocolV3::listDir().