Check Postgres status by checking return code of pg_ctl

Summary:
The current code which was checking the standard output of "pg_ctl status",
doesn't work with non-English locale, causing Akonadi to leave Postgres
running after shutdown, because it thinks the db is no longer running.

BUG: 386173
FIXED-IN: 5.11.3

Reviewers: #kde_pim, mlaurent

Reviewed By: #kde_pim, mlaurent

Subscribers: mlaurent, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D21871