Fix bug introduced in commit 13056b517c0b7d52fe63fd7a8685acca742481bf

https://github.com/cguimaraes/OpenTFTP/commit/13056b517c0b7d52fe63fd7a8685acca742481bf
Initialize TFTP options parameter in TFTP messages
Fix handlers in server session
Support TFTP OACK message in TFTP socket