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

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.

Reviewed by: 0mp, netchild
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D53754

(cherry picked from commit 7861d051de2ea2f244bcf73111a04389dc5bcf51)