Commit  11132fc  assumed that the value
of 0 is never used in practice and thus used it to indicate "disabled",
however that assumption has turned out to be wrong because ostree uses
precisely that value as mtime in inodes, which in turn breaks existing
workflows in this space (see the associated ticket).
Fix this by reverting the above commit (except leaving source_date_epoch
initialized to 0, to prevent GCC warnings as mentioned in that commit).
As to why not just initialize source_date_epoch to -1: time_t happens to
be a signed integer on most platforms but POSIX doesn't specify its
signed-ness.
Add some accompanying tests too.
Fixes: rpm-software-management#2679