Use full path to ifconfig/route binaries in VNET config example script

Before this change, the jail.conf example for VNET jails produces the
following error when running "service jail start vnet":

Starting jails: cannot start jail "vnet":
jail: vnet: exec ifconfig: No such file or directory
jail: vnet: ifconfig epair154 create up: failed

The solution is to provide full paths to both ifconfig and route
(/sbin/ifconfig and /sbin/route, respectively) in the example.
Afterwards, the jail starts without errors. This seems to be the only
occurance in the chapter since the other examples do not have any
exec.{prestart,start,poststop} sections.