Use the new copy timestamp clamping mechanism from the previous commit
to fix unreproducible wall-clock files in the generated images.
That covers both modes: online mode writes through the kernel, and
offline mode stages the tree in a temporary directory before handing it
to mkfs or mcopy. `mcopy -m` then carries the clamped mtimes into the
FAT.
This complements commit 34c3d57474 (directory times) and commit
f191ca982c (FAT insertion order).
Cover all combinations (online/offline, with and without epoch) in the
new `testcase_vfat_reproducibility()`. Update `testcase_make_symlinks()`
to cover the symlink creation/re-fix their parent directory code paths.
Sadly, images are still not reproducible byte for byte. Document
systemd-repart's $SOURCE_DATE_EPOCH handling and remaining
reproducibility issues in the manpage.
Martin Pitt
94bb5f8e4d