Support e.g. eups declare -t XXX -r DDD PROD VERSION

In general, if we are changing a definition and also declaring a tag,
generate a tag:XXX version with the changed attributes
Make "eups unsetup" cleanup all eups shell variables
Ignore "-t newest" tags in ~/.eups unless that's the only version available
Added "version!" to the vro; like "version" except doesn't disable --keep

Motivation:  it's useful to set an absolute version even when you want
to keep already setup products.  The absolute path specification is also useful,
so here's a way to support the latter.  E.g.
   setupRequired(hscPipe --vro version 2.10.0b_hsc)
   setupRequired(astrometry_net_data --vro version! sdss-dr8)
Renamed "newest" tag to "latest".

Maybe it's just an Real English thing, but "newest" just sounds wrong.