rtsol/rtsold: Add option to skip random delay

In accordance with a SHOULD in RFC 4861, rtsol and rtsold wait a
random time between zero and one (aka MAX_RTR_SOLICITATION_DELAY)
seconds before sending a Router Solicitation, in order to avoid
network congestion if many hosts come online at once. (The
question of how many hosts would be required to cause congestion
by each sending a single packet on a Gbps+ network is left to the
reader.)

The new option -i disables this wait and instructs rtsol and rtsold
to send the Router Solicitation immediately.

Reviewed by: bz, kp (earlier version)
MFC after: 1 week
Relnotes: yes
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D32956