We enable symlink support using the --links command line switch.
When symlink support is enabled, the mount backends will translate
the name of the vfs symlinks files (truncating their rclonelink suffix).
Also, operations like rename, symlink etc does not needs the rclonelink
suffix, it is handled internally to pass it to the underlying low level
VFS.
When symlink support is disabled, Symlink and Readlink functions will
transparently manage ".rclonelink" files as regular files.
Fixes#2975
Before this change, the current working directory could disappear
according to the Linux kernel.
This was caused by mount returning different nodes with the same
information in.
This change uses vfs.Node.SetSys to cache the information so we always
return the same node.