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.

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