Add a mask on the qos_flows miss configuration value.

Add a mask on the qos_flows miss configuration value.

The reason for this is to allow the preserved mark/TOS value from the
server to be altered slightly rather than overwritten completely.

Example usage. The following will preserve the netfilter mark, but will
ensure that the (9th) bit specified in the miss value will be set to 1
in the preserved mark:

  qos_flows mark miss=0x100/0xF00