repart: add 'fsverity' flag for CopyFiles= lines

repart: add 'fsverity' flag for CopyFiles= lines

We currently pass the CopyFlags that we use to populate the temporary
directory in the form of a constant at each of the copy_tree_at() call
sites.  De-duplicate that and move it into the `CopyFilesLine` struct,
initializing it from the parser.

Add our first non-constant flag: `fsverity=`.  This can be set to `off`
(the default) or `copy`, in which case we copy the fs-verity state from
the source files.

This arrangement is amenable to the introduction of more flags to
`CopyFiles=` lines, if we want to add them in the future.

Update the `repart.d(5)` manpage.

Closes #35352

Signed-off-by: Allison Karlitskaya <allison.karlitskaya@redhat.com>