fork_worker: factor out synced worker output mirroring

fork_worker: factor out synced worker output mirroring

When running in sync (CLI environment) we mirror the workers output
to both, STDOUT and th task log file, a similar function as the unix
comand line tool tee provides, thus we borrow its name for the
factored out sub method.

This moves ~60 lines of code out of the big fork_worker sub and makes
it easier to read track what happens there.

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