support maxfeatures for GetObservation requests

[tomkralidis]

SOS is in the same predicament as WFS w.r.t. voluminous XML responses.

Supporting ows|sos_maxfeatures would be valuable for service providers to control output for GetObservation requests/responses.

Subsequently, msSOSGetCapabilities should check if MAP/WEB/METADATA/???_maxfeatures is set. If it is, then the following will be written out under ows:OperationsMetadata.

<ows:Constraint name="DefaultMaxFeatures">
 <ows:Value>$mf</ows:Value>
</ows:Constraint>

...where $mf is the value of MAP/WEB/METADATA/???_maxfeatures

[tomkralidis]

• implemented in r6971. If sos_maxfeatures is set, msSOSGetObservation() breaks when threshold is reached. Also, msSOSGetCapabilities outputs an ows:Constraint element
• HISTORY.txt updated
• msautotest: no applicable changes required
• documentation: sos_maxfeatures added to ​http://mapserver.gis.umn.edu/docs/howto/sos_server