WMS: Add WARNING comments in capabilities for missing mandatory fields

[dmorissette]

WMS: Add <!-- WARNING: ... --> comments in capabilities for required 
  items that cannot be generated by lack of metadata values, etc.

[dmorissette]

Note1: Try to provide default in addition to the warning where it makes sense, 
like for the Service Title

[dmorissette]

SRS tags are required:

> Below are the
> normative comments from the V1.0.7 dtd that provide a more specific
> description of what is required (normative comments were removed from the
> V1.0.8 dtd.
> 
> <!-- Listing of available Spatial Reference Systems (SRS).
> * The root Layer element must include a list of all SRSes which
>   are common to *all* subsidiary layers.  This allows clients
>   to quickly determine that a server cannot possibly satisfy a
>   request for a particular SRS.  Use an empty element if there
>   is no common SRS.
> * Layer-specific SRS: Optionally, layers may add to the global SRS list, or
> to
>   the list inherited from a parent layer as described above.
>   Any duplication should be ignored by clients.  That is, a particular
>   layer may list all of its SRSes, even if it repeats
>   information in the top-level SRS.

[dmorissette]

Added Warnings and defaults when applicable for missing mandatory metadata and 
mapfile parameters.