Add check-fstab-mounts plugin (Linux only)

This plugin parses `/etc/fstab` and `/proc/mounts` to ensure that all
mountpoints specified in `fstab` are currently mounted.  Optionally
filtered by filesystem type.
Add iostat-full-metrics plugin

Both the currently existing iostat plugins were borken in some way
(iostat-extended-metrics produces no output, and iostat-metrics
erroneously outputs headers as metrics).  Additionally neither of them
includes the `avg-cpu` stats, which can be useful for
alerting/diagnosis.

This plugin has more robust parsing (should not break with banners,
minor formatting changes, or added/removed fields), and includes
`avg-cpu` stats.  Included fields are dynamicall generated from output.
Update the netif-metrics plugin for efficiency

The `sar` call is really expensive, so avoid calling it twice, and do
all processing in Ruby
Fix iostat call to poll twice (first result is since boot, not now)
Pull sensu-community-modules subtree