rtsol(d): add script for "M bit"

While we do support the "O bit" running a script (usually to start a
dhcpv6 client) we have no options for setups which set the "M bit" for,
e.g., static address assignment as in EC2.

Duplicate most of the "O bit" logic to also start a script for the
"M bit" with the one difference: if the "M bit" is set we will not
start the script for the "O bit" as well (per RFC 4861, Section 4.2).

Reviewed by: hrs, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26099