MFC r293781:

Restore uart PPS signal capture polarity to its historical norm, and add an
option to invert the polarity in software. Also add an option to capture
very narrow pulses by using the hardware's MSR delta-bit capability of
latching line state changes.

Relnotes: yes

MFC r286469, r286591, r286595, r286596, r286613:

Provide the tty-layer mutex when initializing the pps api. This allows
time_pps_fetch() to be used in blocking mode.

Allow the choice of PPS signal captured by uart(4) to be runtime-configured,
eliminating the need to build a custom kernel to use the CTS signal.

Correct the polarity of the PPS assert and clear events with respect to the
electrical signals on the serial port.

Document the change in polarity of the uart(4) PPS capture.

Style fix -- do the braces for switches correctly.

Relnotes: yes

MFC r286469, r286591, r286595, r286596, r286613:

Provide the tty-layer mutex when initializing the pps api. This allows
time_pps_fetch() to be used in blocking mode.

Allow the choice of PPS signal captured by uart(4) to be runtime-configured,
eliminating the need to build a custom kernel to use the CTS signal.

Correct the polarity of the PPS assert and clear events with respect to the
electrical signals on the serial port.

Document the change in polarity of the uart(4) PPS capture.

Style fix -- do the braces for switches correctly.

Relnotes: yes