Add error info for ftruncate() call.
Implement some cluster events.
Implement cluster events for comments.
Implement cluster events for downtimes.
make: mkembedconfig is required for 'dist'

fixes #4489
Build fix for --enable-debug.
Fix a grammar mistake in --help.
Revert "make: mkembedconfig is required for 'dist'"

This reverts commit 1e637dbbd40d1e64f32567cd44230980559b3a10.
Update icinga2.spec

Added several "BuildRequires" that were needed to make the build process work.

Also added a "sh autogen.sh" to the %build section, otherwise there is no ./configure to execute.

Signed-off-by: Michael Friedrich <michael.friedrich@netways.de>