This makes the host CPID stable; if you repeatedly install BOINC
on a particular node, it will get the same host CPID each time,
and your host table won't get lots of redundant entries.
A host can have multiple NICs;
we use the MAC address of the first Ethernet controller we find,
or the last NIC if there is none.
Of course, this will create problems if we get the same MAC address
for different hosts; in principle this shouldn't happen.
Remove the unused file hostinfo_network.h