Add behaviour control for nil field value handling

[stevenh]

Add concept of a WMI Client which allows the consumer to control how nil values are handled.

By default this maintains compatibility with the original implementation which:

• Only allows nil values for pointer field types
• Converts nil pointer values to the types zero value
• Errors if a struct field is missing from the result set

Creating a custom client allows the configuration of how:

• Pointer fields handle nil values (PtrNil)
• None pointer fields handle nil values (NonePtrZero)
• Missing fields are handled (AllowMissingFields)

The missing field option is useful as it allows the full struct definition to be used with custom queries that only return a partial set of information which they require, instead of having to create an alternative struct only those fields.

Also fix miss-match errors information returning "" vs "*" for pointer fields which can be confusing.

[darkliquid]

:+1: