groupstats.pl: Refactor query handling.

Don't misuse variables:
- Push other binding vars to (new) @Params instead of
  filling up @GroupList with things it wasn't meant for.
- Replace generic @QueryPart by @QueryGroupList and
  @QueryThreshold, then build $WhereClause from that.
- Don't use $OrderClause for setting level (more or
  less than x), use (new) $Level instead.

Signed-off-by: Thomas Hochstein <thh@inter.net>