Pull request: all: imp install script logs, systemd unit file

Updates #3579.

Squashed commit of the following:

commit 9a25fe15c999b351639d4baaa556c0b6e52c3102
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Sep 22 18:09:25 2021 +0300

    all: imp install script logs, systemd unit file

scripts/install.sh

@@ -451,7 +451,7 @@ unpack() {
 		;;
 	esac
 
-	log "successfully unpacked, contents: $( echo; ls -l -A "$out_dir" )"
+	log "successfully unpacked, contents: $( echo; ls -l -A "$agh_dir" )"
 
 	rm "$pkg_name"
 }