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.

The historical UART_PPS_ON_CTS kernel option is still honored, but now it
can be overridden at runtime using a tunable to configure all uart devices
(hw.uart.pps_mode) or specific devices (dev.uart.#.pps_mode). The per-
device config is both a tunable and a writable sysctl.

This syncs the PPS capabilities of uart(4) with the enhancements recently
recently added to ucom(4) for capturing from USB serial devices.

Relnotes: yes