Some devices take undesired actions when RTS and DTR are
asserted. Some development boards for example will reset on DTR,
and some radio interfaces will transmit on RTS.

This patch allows "stty -f /dev/ttyu9.init -rtsdtr" to prevent
RTS and DTR from being asserted on open(), allowing these devices
to be used without problems.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D20031