Share the NFS connection with the NFS input plugin.

Read all directory entries into memory and close the struct nfsdir
before returning the StorageDirectoryReader instance.  This is what
libnfs does, anyway.

Creating a NfsStorage sets its own export_name as the "base".  Now
NfsFileReader can use this information to derive the export_name to be
mounted, instead of guessing.  This solves the "too many connection"
problem on the NFS server while updating the database.

"override" implies "virtual".

Assume the configured filesystem character set is also used by the NFS
server.

Swap the order of adding the URI and the slash, because appending is
cheaper than inserting.