Add parameter for port scan duration

[samkopmann]

Enable steahly scans with random destination ports (over the complete range) with limited durtaion.

[pepper-jk]

Looks good. Please squash into one commit with the commit message of the following format:

Port Scan: Add parameter for scan duration

Enable stealth scans with random destination ports with short duration.

Also feel free to add more description, if you like.