copy: add COPY_SYNCFS flag

copy: add COPY_SYNCFS flag

When copying large directory trees it should be a better idea to sync
the whole fs once when we are done instead of individually for each
file, hence add COPY_SYNCFS.

As opposed to COPY_FSYNC/COPY_FSYNC_FULL this only really applies to the
top-level directory, after completion of the whole copy.