Support 'unlink' option

[z3ntu]

[TQFTP] RRQ: /readwrite/mcfg.tmp (mode=octet rsize=0 seek=0)
[TQFTP] Remote returned END OF TRANSFER: 9 - End of Transfer
[TQFTP] WRQ: /readwrite/mcfg.tmp (octet)
[TQFTP] RRQ: /readwrite/mcfg.tmp (mode=octet rsize=0 seek=0)
[TQFTP] Remote returned END OF TRANSFER: 9 - End of Transfer
[TQFTP] WRQ: /readwrite/mcfg.tmp (octet)
[TQFTP] Ignoring unknown option 'unlink' with value '0'

I assume the modem wants us to delete (=unlink) the file again when the option is present. Doesn't seem to cause any unexpected issues not doing this for mcfg.tmp right now so I haven't implemented it yet.