This new vfs_import plugin allows a VFS instance to be populated during
construction using a sub-VFS configured in an '<import>' configuration
node. This allows the ram_fs File_system server to be replaced by the
VFS server by reimplementing the ram_fs 'content' feature. At the
moment the copying of symlinks is not enabled, and the resources
obtained by the import file-system may not be freed after the import is
finished.
Fix#2906
