interface plugin: Fix for bogus interfaces under Solaris.

interface plugin: Fix for bogus interfaces under Solaris.

Under newer versions of Solaris, the method for finding network interfaces
results in 'collectd' gathering statistics on an interface called 'mac', along
with 16 interfaces named 'wrsmd0' through 'wrsmd15' (which are for WCI Remote
Shared Memory). The 'mac' interface is especially problematic because it
occurs multiple times, which results in warnings from rrdtool about insertions
with the same timestamp.

Change-Id: I1ae070ecbc820da25c0f0ad457aba80213339fd2
Signed-off-by: Florian Forster <octo@collectd.org>