Debian: Added checkconfig helper to initscript.

And simplified check handling over all functions.

Refs #5305
Debian: Cleaned up indention in init script.

Refs #5305

Conflicts:
	debian/icinga2-common.icinga2.init
Fix indentation in lib/icinga/externalcommandprocessor.cpp
Debian: Added foreground init helper to debug Icinga 2.

Refs #5305
Merge branch 'feature/debian-init-helpers-5305' into next

Fixes #5305