and simplify callsites to use it instead of the current
    "seek to end, find pos, seek back to original pos"
pattern

Change-Id: Ib5828868f73c341891efc759af8bd4695ae2f33c
Reviewed-on: https://gerrit.libreoffice.org/61738
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>