MFC r279728, r279729, r279756, r279773, r282424, r281367:

MFC r279728, r279729, r279756, r279773, r282424, r281367:

  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.

git-svn-id: svn://svn.freebsd.org/base/stable/10@283341 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f