Proof of concept how a real reload-handling with systemd could work.

Refs #6118
Unlink the pidfile when we really shutdown (not reload).

Refs #6118
Add Process::GetPID and make it work on Unix and Windows.

Refs #6118
Merge branch 'feature/systemd-reload-6118' into next

Fixes #6118
Documentation: Classic UI command expander feature does not work using Icinga 2.
Remove duplicate information in INSTALL file.
Fix deadlock in db_ido.

Fixes #6230
Test.