Add mutex support to the pps_ioctl() API in the kernel.

Add PPS support to USB serial drivers.

Use correct mode variable for PPS support.

Switch polarity of USB serial PPS events.

The ftdi "get latency" and "get bitmode" device commands are read
operations, not writes.

Implement a mechanism for making changes in the kernel<->driver PPS
interface without breaking ABI or API compatibility with existing drivers.

Bump version number to indicate the new PPS ABI version changes in the
pps_state structure.