files: make OC\Files\Storage\Local WORM friendly

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>

lib/private/Files/Storage/Local.php

@@ -274,6 +274,8 @@ public function file_get_contents($path) {
 	}
 
 	public function file_put_contents($path, $data) {
+		// support Write-Once-Read-Many filesystems
+		$this->unlink($path);
 		return file_put_contents($this->getSourcePath($path), $data);
 	}
 
@@ -353,6 +355,10 @@ public function copy($path1, $path2) {
 	}
 
 	public function fopen($path, $mode) {
+		if ($mode === 'w') {
+			// support Write-Once-Read-Many filesystems
+			$this->unlink($path);
+		}
 		return fopen($this->getSourcePath($path), $mode);
 	}