tag helpers: add get_unique_tags method for filtering out duplicates

tag helpers: add get_unique_tags method for filtering out duplicates

tags must be unique, allow the user some control in how unique (case
sensitive) and honor the ordering settings (even if I doubt any
production setups wants to spent time and $$$ on cautiously
reordering all tags of their dozens to hundreds virtual guests..

Have some duplicate code to avoid checking to much in the loop
itself, as frequent branches can be more expensive.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>