Make Npcap and Npcap OEM installer options case insensitive

[fyodor]

Given that the Npcap (and Npcap OEM) self-installers don't have many options, we are probably not going to ever want arguments that differ only in case. So there is probably no reason to require the "correct" case as we do now. We've manually done this for /s so it works like the canonical /S (silent install) option. But we should look for an easy way to make NSIS do this with all options.

Related issue for Nmap: nmap/nmap#2521

[fyodor]

So we manually did this for silent-install (/S and /s) but kept this issue to ensure all other installer options are case-insensitive. But @dmiller-nmap did some testing and reports that they already are (except for /D which we can't do due to NSIS complications) and so we are happy to close this issue as complete!