Remove etc/rc.d/mrouted

mrouted has been available in ports for the last 8 years as net/mrouted . An
equivalent rc.d script has been present in the port.

Remove all corresponding variables from etc/defaults/rc.conf

Relnotes: yes
Followup to r275299: add an ObsoleteFiles.inc entry for etc/rc.d/mrouted
Unbreak the code for non-digits below '0' by casting the expression
to unsigned int.

Pointed out by: bde