Decide if/how Nmap should handle Npcap re-installs -- and then implement it #2449

With user-mode Windows library and component applications, it is common for multiple copies to be on a system since each application which needs them often installs their own. This has its problems, but can also give each application extra control of its components. For system device drivers like Npcap, this approach is generally not efficient or practical. Applications share one Npcap driver like they would the lower level network adapter device driver or a storage system device driver. Since multiple applications may share an Npcap install, it is also possible that another application might uninstall Npcap (maybe while that application itself is being installed) even though Nmap may still be on the system depending on Npcap. The best approach might be for Nmap to unpack an Npcap (or Npap OEM for Nmap OEM) installer so it can be reinstalled if necessary. We will have to decide where it does the check and (if necessary) re-install. We also should probably check whether that copy of Nmap installed Npcap in the first place (it is possible to use Nmap without Npcap) and whether the user running Nmap has the privileges available to install Npcap OEM on the system.

A related issue is whether we should or do uninstall Npcap when Nmap is uninstalled, given that other applications might possibly be using Npcap. It may be best to just document that uninstalling Nmap involves two steps: uninstall Nmap and then uninstall Npcap. Both can be done through the Windows add/remove software panel. As of Nmap 7.92, the Nmap uninstaller does not touch Npcap to avoid hosing any other software which might depend on it. Users who want to uninstall Npcap can do it themselves through add/remove programs.

Besides being important for Nmap itself, Nmap's behavior can potentially serve as a model for other software which uses Npcap, such as Wireshark. We should probably add the best practices we decide on to the Npcap documentation.