savecore: add missing call to cap_openlog when in capabilities mode

Signed-off-by: Stéphane Rochoy <stephane.rochoy@stormshield.eu>

Reviewed by: markj, oshogbo
MFC after: 2 weeks
Pull Request: https://github.com/freebsd/freebsd-src/pull/1546

(cherry picked from commit c901e89d8e467b56f844ab5dba18dc20d503c2be)