[kconf_update] move from custom logging tech to qCDebug

Summary:
the previous logging tech got kind of defunct in 2014 (to the point where
it was basically qCDebug). seeing as no one really complained it seems
reasonable to just move to qCDebug instead and make use of category filters
and other qdebug goodness (such as system logging facilities for the
various platforms)

• new logging category kf5.kconfig.update; at info level by default (i.e. unless otherwise configured kconf_update is now silent)
• --debug cmdline option now also attempts to force-enable the debug mode on that category (and debugs that attempt in of itself, so we don't get confused by categories magically getting enabled).
• all log() calls are now qCDebug calls
• all logFileErorr() calls (which was context-sensitive to the .upd file parsing) have been changed to qCDebugFile
• qCDebugFile is a new *macro* wrapping around qCDebug to give it file context
• everything is now qCDebug instead of qDebug
• arguments updated to drop excess quoting and spaces to reduce "noise"

https://markmail.org/thread/ofaeqcabguxyohyb

Test Plan: updater still works and debugs when debug is enabled

Reviewers: apol, aacid, Frameworks

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19508