Improve Linux performance of GetAllInterfaces

[wjordan]

This Linux-specific optimization avoids a separate Netlink dump of addresses for each individual interface, which is prohibitively slow on servers with a large number (thousands) of network interfaces.

Here are some example test results on a sample server using the sockaddr eval command (which provides sockaddr.GetAllInterfaces() as template data), showing a ~100x performance improvement:

$ ls /sys/class/net | wc -l
3202

$ time ./sockaddr-orig eval 0
0

real	1m3.956s
user	0m36.514s
sys	0m55.716s

$ time ./sockaddr-opt eval 0
0

real	0m0.457s
user	0m0.096s
sys	0m0.443s

See golang/go#53660 for further discussion of this Linux performance issue with net.Interface.