etc/defaults/rc.conf: Add -i flag to rtsol/rtsold

This disables the random (between zero and one seconds) delay before
rtsol and rtsold send a a Router Solicitation packet. This delay is
specified as a SHOULD by RFC 4861 for avoidance of network congestion,
but network speeds have increased enough in the 25 years since this
first appeared (in RFC 1970) that it seems unnecessary as a default
at this point.

This speeds up the FreeBSD boot process by an average of 500 ms.

Reviewed by: kp
MFC after: 1 week
Relnotes: yes
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D33089