repart: add new --copy-source= switch

repart: add new --copy-source= switch

This specifies a directory to which CopyFiles= is considered relative.
If unset defaults to the --root=/--image= setting, or host / otherwise.

This is very similar to --root= but is much more focussed: it is really
and exclusively about CopyFiles= (and related settings such as
ExcludeFiles=) and does not affect any of the settings, i.e. it doesn't
affect CopyBlocks=, the machine ID/seed handling, or where definitions
are read from.

In fact, --root= and --copy-source= may be combined for example to
use the machine ID and similar from one tree, but the copy the files
from another.