(maint) Redirect dmidecode stderr to /dev/null

Previously most calls to dmidecode had stderr redirected to /dev/null.
dmidecode tends to have error messages that go to stderr during execution,
which pollutes facter runs. This commit updates the remaining calls to
dmidecode to also redirect stderr to /dev/null so that facter runs can continue
in blissful ignorance of any dmidecode errors.
Merge pull request #596 from haus/stable

(maint) Redirect dmidecode stderr to /dev/null