Things were simple back when all Linux called them "ethN" and there were
very few pseudo-devices to worry about...

Now we find the "active" experimental interfaces from the Emulab-provided
/var/emulab/boot/tmcc/ifconfig info. We use the MACs there along with "findif"
to identify interfaces of interest. N.B.: as it stands, this version of
slothd requires the most recent change to "findif" to accept command line
MAC addresses with ':'s.

Also fixed a couple of bugs while in here. The "netstat" command output
(used to obtain packet counts on both Linux and FreeBSD) has also changed
over the years. At some point, the Linux version (at least on CentOS) removed
the "Met" (metric) column from the output, throwing off the parsing of input
and output packet counts. For FreeBSD, an "Idrops" column was added which
throws off the output packet count. Both are now handled by noting whether
the columns exist when first seeing the header, and adjusting the sscanf
format accordingly. On the Linux side I have only seen this affect CentOS 7.
For FreeBSD, it affects all versions back to at least FreeBSD 8!