Adding cf_util_get_strings()

The new function is a refactor of cf_util_get_string() to support
multiple string pointers. It also validates the number of strings in the
config by using a given parameter (expected_strings).
ipmi plugin: Refactor to improve the config parser

The new config parser uses the complex strategy. It also makes use of
the new cf_util_get_strings() function to parse config elements with
multiple string values. E.g.:

   SensorAlias  "default sensor name" "aliased name"

It also changes the previous behavior of overloading the Sensor option.
Now, the aliases are segregated into their own SensorAlias option. It
makes the config a bit more verbose but it's much clearer in its usage.
ipmi plugin: Fixing the documentation