Run netoptions startup script in vnet jails.

People use rc.conf inside vnet jails to configure networking setups.
Presumably because some sysctl were not virtualised up until r356527 the
script was not run for vnet jails leaving the rc.conf options without
effect for non-obvious reasons. Run the netoptions startup script also
for VNET jails now to make the rc.conf options work.

PR: 243193
MFC after: 2 weeks