munin-node passes the original interface name (as extracted from
/proc/net/dev) to if_* and if_err_*. Thus replacing dash with underscore
results in interfaces not being found anymore.

Closes: #1927

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan McCrohan <jmccrohan@gmail.com>