export: bring matched items up to date on every run

Export matched each item against the target and then skipped it, so a second
run -- the usual final pass of a cutover -- never carried anything that had
changed at the source since the first: read and flagged state, moves between
folders, edited contacts, events and Sieve scripts. It reported them as
skipped and exited 0, while the usage guide said matched items were updated.

Matched items are now updated, with one batched /set per type:

- Email: keywords are set to the archive's, added and removed, compared
  case-insensitively. Memberships of folders this run migrated are added and
  removed to match; folders that exist only on the target are left alone,
  and a message is never left in no folder. Properties the server did not
  report are not touched.
- Contacts and events: when both copies carry `updated`, the archive's is
  written only if it is newer, compared as instants so an offset or a
  fraction of a second is not taken for a change; otherwise each property the
  archive writes is compared, and those that differ are sent whole.
- Sieve scripts: the target's copy is downloaded and compared byte for byte
  with what export would write -- after renaming Stalwart's vendor names for
  an inbuxa target -- and replaced when it differs, so a renamed script is
  not re-uploaded on every run.

Updated items are counted as `updated`; unchanged ones stay `skipped`. The
usage guide now describes this.

docs/usage.md

@@ -211,16 +211,23 @@ inbuxa-migrate export \
 
 Writes `ARCHIVE` into an account on a JMAP server, usually inbuxa. It keeps no
 state of its own: every run matches the archive against the target afresh.
-By default it only adds: what the archive holds and the target lacks is
-created, and anything already on the target is left as it is -- an item that
-matches is not updated yet, so a change made at the source after the first
-export does not reach the target on a second one.
+By default it only adds and updates: what the target lacks is created, what
+it has is brought up to date, and anything on the target that the archive
+does not cover is left alone. So a second run after a later import carries
+what changed at the source in between.
 
-Email is matched by Message-ID, or without one by sender, subject, date and
-recipients; where several messages share one, size decides. A message the
-source kept in several folders -- IMAP and Maildir copies, Gmail labels -- is
-written once, in all of them, and a later run adds any folder it is still
-missing on the target.
+- **Email** is matched by Message-ID, or without one by sender, subject, date
+  and recipients; where several messages share one, size decides. A message
+  the source kept in several folders -- IMAP and Maildir copies, Gmail labels
+  -- is written once, in all of them. On a match, its keywords (read,
+  flagged and the rest) are set to the archive's, and so are its memberships
+  of folders this run migrated. Folders that exist only on the target are
+  left alone, and a message is never left in no folder.
+- **Contacts and events** are matched by UID. When both copies carry an
+  `updated` time, the archive's is written only if it is newer; otherwise
+  the properties that differ are written.
+- **Sieve scripts** are matched by name, and the target's is replaced when
+  its content differs from the archive's.
 
 `--prune` also deletes what is on the target and not in the archive. It asks
 first; `--yes` answers for it, for scripts. Export speaks JMAP only.