Update http-screenshot.nse

[d7x]

Wait for the scan to complete and put a screenshot in /tmp/ using gnome-screenshot, use 64-bit wkhtmltoimage.

[PolarBearGod]

Just following this repo but was curious:
The post operation uses Gnome Screenshot which is not universal across all *nix desktop environments - is there any particular reason to add it considering all of the other DEs?
Also why forcefully use the 64 bit version of wkhtmltoimage? i386 works on both x86 and x64 architecture giving better cross capability.

[d7x]

@PolarBearGod the changes proposed are not the best option regarding compatibility, but these are the changes I made because of the screenshot not waiting for the scan to complete on slow scans. The main change is the post rule added, and it could actually still use wkhtmltoimage. The changes suggested are mostly suited to the average environment used and per my own preference, so for compatibility the gnome-screenshot could be substituted with the original author's decision.