scan will maintain a persistent database of file signatures
(default /var/lib/sfdupes/db.sqlite, overridable via
SFDUPES_DATABASE) that survives between runs; rescans hash only new
or changed files (mtime/size) and remove records for vanished files,
so scan can be cronned daily. report and trees will read the
database instead of a scan stream.
scan now takes one or more PATH operands (directories or regular
files) via cobra flags instead of the -root flag with its /srv
default; invoking scan with no operand is a usage error and a
nonexistent operand is fatal. Filesystem boundaries are crossed by
default; the new -x/--one-file-system flag (GNU du/rsync convention)
stops the walk at each operand's filesystem, implemented by comparing
lstat device IDs with build-tagged helpers for darwin's int32 Dev.
Verified against a real mounted disk image: default crosses, -x does
not, --one-file-system is identical to -x.
