add service and layer level description

[tomkralidis]

1./ service (web) level, between <metadataLink> and <name>:

<description>[ows_abstract]</description>

2./ layer level, between <metadataLink> and <name>:

<description>[ows_abstract]</description>

..I've put [ows_abstract] as the mapfile metadata name because it can double up
to show up for other services, but you may want to use [wcs_description].  Your
call.

[tomkralidis]

This is now in mapwcs.c.  Thanks.