Added ability to specify CVS symbolic tags that need to be imported as Perforce labels
* convdir/config can define WANTTAGS and list of CVS symbolic tags
* bin/genmetadata generates list of CVS symbolic tags in convdir/labels.lst,
which can be used to configure the WANTTAGS list in convdir/config
* if WANTTAGS is defined,
bin/dolabels only imports the CVS symbolic tags listed in convdir/config