Allow transmit packet bufring in software to be disabled.

• Add new sysctl node to control the transmit packet bufring.

• Add optimised version of the transmit routine which output packets

directly to the DMA ring instead of using bufring in case the transmit
lock is congested. This can reduce the number of taskswitches which in
turn influence the overall system CPU usage, depending on the
workload.

• Add " TX" suffix to debug name for transmit mutexes to silence some

witness warnings about aquiring duplicate locks having same name.

MFC after: 1 week
Sponsored by: Mellanox Technologies
Suggested by: gallatin @