(FACT-459) Fix MTU interface facts not being reported for Archlinux.

The regular expression being used assumes that MTU is uppercase and
followed by a colon. On Archlinux, ifconfig (net-tools 1.60+) reports
a lowercase "mtu" followed by a space.
Merge pull request #667 from peterhuene/fact/459

(FACT-459) Fix MTU interface facts not being reported for Archlinux.