(maint) Return nil on dns command lookup fail

This updates the domain fact to ignore the output of nonexistent
commands, rather than raising an error. While it would be better to
refactor the domain fact into smaller resolutions that can be confined
based on the presence of the command that's a more ambitious refactor
than what I can do right now.
Merge branch 'maint/stable/solaris-missing-domain-commands' into stable
Merge branch 'stable' into facter-2
Merge branch 'facter-2'
(maint) Return nil on sysctl command lookup fail

See also 7fc668519f8e.
Merge branch 'maint/stable/solaris-missing-sysctl-command' into stable

One would think I would just fix all of these issues in a single branch.
One would be more reasonable than I.
Merge branch 'stable' into facter-2
Merge branch 'facter-2'
(maint) Return nil on uptime command fail
(maint) Return nil on zoneadm command fail
Merge branch 'maint/stable/solaris-missing-commands' into stable
Merge branch 'stable' into facter-2
Merge branch 'facter-2'
(maint) Restrict rackspace facts based on executable
Merge branch 'maint/master/is_rsc-command-missing'
(FACT-379) Don't return VLANs when only header is present

On CentOS 5 the vlans file is always present regardless of if any vlans
is defined, which behaves like there are VLANs defined but doesn't match
any regexps. This commit explicitly ensures that we have a non-empty
list of VLANs before returning anything.
Merge branch 'bugfix/facter-2/fact-379-centos-5-vlans' into stable
Merge branch 'stable' into facter-2
Merge branch 'facter-2'
(packaging) Update FACTERVERSION to 2.0.1-rc2