Export a sysctl count of RX FIFO overrun events.

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.

Reviewed by: brooks
MFC after: 2 weeks
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D24368