sysrc: Fix check flag logic for append and subtract

When using sysrc with the check flag (-c), the append (+=) and subtract
(-=) operations result in incorrect return values because on the check
path the necessary union/difference calculation logic is not performed.

However, the correct union/difference calculation is already performed
when running without the check flag. We fix the issue on the check path
by using the results from the existing union/difference calculation in
the check logic to get the correct return values.

PR: 279200
Reviewed by: markj
MFC after: 1 month
Pull Request: https://github.com/freebsd/freebsd-src/pull/1664

(cherry picked from commit c97460c4d97db1bd46dab9f2bdbc90cd1ed7757f)