Leave information on scanned hosts

This routine stores information about the scan on the scanned host, provided it is a unixoid system offering ssh access with a standard shell. The information cover hostname, scan start time and scan end time. No details about the actual scan results are stored on the scanned host. By default, this routine is disabled even it is selected to run. To activate it, it needs to be explictely enabled with its corresponding preference switch. The preference 'Message' may contain 3 placeholder where respective content will be inserted into the message when the message is finally created on the target system: '::HOSTNAME::', '::SCAN_START::' and '::SCAN_STOP::'. Two methods are offered (one or even both concurrently can be used): * Syslog: The utility 'logger' on the target system is used to issue the message. The message will appear in the standard log environment as configured on the corresponding target system. Error is reported in case the logger utility is not available. * File: A filename under /tmp can be choosen where the message is left. It is configurable to either overwrite the file each time or to append new information. A token is added to this file to ensure only files created by this routine are used. Error is reported when the access rights are not sufficient or symbolic links detected.