File Stream API now unlinks before open with O_EXCL

The File Stream API now unlinks the destination file (i.e.,
`<FILENAME>.cfnew`) before opening it with the `O_EXCL` flag. Previously
the agent would fail if the destination file already exists.
Fortunately, the File Stream API unlinks this file afterwards, both on
success and error, causing the agent to recover. Both the `cf-net get
<FILENAME>` command and the `copy_from` attribute were affected.

Ticket: None
Changelog: Commit
Signed-off-by: Lars Erik Wik <lars.erik.wik@northern.tech>

Author: larsewi

libcfnet/file_stream.c

@@ -853,6 +853,7 @@ static bool RecvDelta(
     char in_buf[PROTOCOL_MESSAGE_SIZE * 2], out_buf[PROTOCOL_MESSAGE_SIZE];
 
     /* Open/create the destination file */
+    unlink(dest);
     int new = safe_open_create_perms(
         dest, O_WRONLY | O_CREAT | O_TRUNC | O_EXCL | O_BINARY, perms);
     if (new == -1)