Empty out any RTTs from the last round.

If we don't empty the hash before starting this round, then if we get
an SNMP error for a target, we'll report his results from the
previous round.
Don't reuse $t, even if perl scoping rules allow it.

Reusing $t to iterate over the ping results is just confusing,
especially since one line later we assign to the other $t.
Use a new locally-scoped $result variable instead.
Improved logging from SNMP errors.
A probe to use the EOS "ping" cli command

Based on OpenSSHJunOSPing, this probe ssh's to a
device and uses the EOS "ping" command to collect
data.
Better error logging, and support unpriv'd accounts
Merge pull request #15 from fenner/master

DismanPing updates: don't return stale data, log more helpful errors, and rename weird variable