rc.subr: Try to make svjc option handling a bit easier to read

[markj]

Summary

Specifically, make this code fit in fewer columns:

• deindent cases to conform to the usual style,
• use a local variable to minimize duplication in each case.

No functional change intended.

[netchild]

Visual inspection: looks functionally equivalent, all options match their intend.
Test run: OK with 31 service jails (at least net_basic works ok).