Document /etc/hosts setup for localhost and loopback addresses

[laeti-tia]

Following the discussion and corrections from perfsonar/toolkit#448 and other related issues, we discovered that the different distros we support don't have the same localhost entries for the loopback interfaces, both v4 and v6. This can be even different depending on what IP support is enabled at installation time.

We should document that to function properly, perfSONAR needs a localhost entry in /etc/hosts pointing at both the IPv4 loopback and IPv6 loopback.

[mfeit-internet2]

One or the other should be sufficient. It occurs to me that we no longer need to be tied to localhost now that I have code to fish out the loopback addresses. The requirement then would be that there has to be at least one loopback address.

[laeti-tia]

localhost is also used in other places than pScheduler, I guess these would need to use your new library function to be independent of that.

[mfeit-internet2]

Wrote an issue on that: perfsonar/pscheduler#1415

The code should be short enough that rubber stamping it into other packages shouldn't be a big deal.