Some filesystems run as a Write-Once-Read-Many storages. This
makes them impossible to use with NexeCloud, as the file system
layers uses `truncate` syscall (through file_put_contents function).

As Nextcloud is never updates existing files, removing the old entry
and creatint a new one on update will allow NextCoud to update on such
file systems.

Update Local#fopen and Local#file_put_contents to remote existing
file before truncating.

Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>

Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>

To avoid extra truncate on non WORM file systems, add a new config
option `localstorage.unlink_on_truncate`, which defaults to false.

The OC\Files\Storage\Local is update to respect that option.

Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>