Mention setcap(8)-fix in EPERM error on Linux

[Ma27]

This is already mentioned in the README, but I figured that it might be
helpful to point linux-users to this if they try to run bandwhich
right after installing it and encountering this kind of error.

[imsnif]

Hey - thanks for this!

My opinion right now is that this might be a little too much. I'd like for our interaction with the user to be as clear and concise as possible. I think the "sudo" suggestion is just there to give most users a way they're familiar with to solve the current problem quickly. But I'm open to be convinced otherwise if you disagree.

[Ma27]

The thing is, when using sudo, the entire program has root permissions and is executed in the root environment (unless you use sudo -E though) while just a few permissions would be granted with the setcap wrapper. Also it might possible then to allow non-sudoers to use bandwhich by creating a bandwhich group and let it own the bandwhich binary.

I agree though that the message might seem a bit bloated. While the macOS-message is fine, I'd suggest something like this for Linux:

nsufficient permissions to listen on network interface(s). Please try one of the following fixes:

* run bandwhich with sudo
* create a `setcap(8)` wrapper for `bandwhich` with the permissions `cap_net_raw,cap_net_admin+ep`

[imsnif]

Sounds good, let's do it.

[imsnif]

Looks great, thanks!