More reliable packages available for RHEL, CentOS, and Fedora

[Jason A. Donenfeld]

It turns out that ELRepo is a bit of a crumbling mess, unfortunately.
FOR CENTOS USERS ONLY, the instructions in the previous email need to
include:

$ sudo yum install yum-plugins-elrepo

Hopefully Red Hat will simply include WireGuard in RHEL 7 and 8, so
that we won't have to deal with this. But for now, this appears to be
the best we've got.

Jason