Export a sysctl count of RX FIFO overrun events.

[jhb]

Summary

uart(4) backends currently detect RX FIFO overrun errors and report
them to the uart(4) core layer. They are then reported to the generic
TTY layer which promptly ignores them. As a result, there is
currently no good way to determine if a uart is experiencing RX FIFO
overruns. One could add a generic per-tty counter, but there did not
appear to be a good way to export those. Instead, add a sysctl under
the uart(4) sysctl tree to export the count of overruns.

Test Plan

• use a uart(4) which is known to drop characters. The sysctl increments when pasting a 20k text file over the uart.

[jhb]

• Add missing file.

[brooks]

sys/dev/uart/uart_core.c

749

Do we need to remove it in detach?

[jhb]

sys/dev/uart/uart_core.c

749

No, new-bus auto-deletes sysctls associated with the device sysctl context returned by device_get_sysctl_ctx() during detach.