service: respect nojailvnet keyword

In the 761d2bb5b9e70cf30f9c2dac62a47a2d2593e83f we added nojailvnet
keyword. The nojailvnet keyword is used to skip startup scripts in
jails that are run without VNET.

The service.sh was omitted in this commit. The service.sh
even documents that this is the same code as in rc - so lets reflect
that.

Submitted by: Adam Wołk <a.wolk@fudosecurity.com>
Sponsored by: Fudo Security