OpenZFS 8940 - Sending an intra-pool resumable send stream may result in EXDEV

Because resuming from a token requires "guid" -> "snapshot" mapping
we have to walk the whole dataset hierarchy to find the right snapshot
to send; when both source and destination exists, for an incremental
resumable stream, libzfs gets confused and picks up the wrong snapshot
to send from: this results in attempting to send
   "destination@snap1 -> source@snap2"
instead of
   "source@snap1 -> source@snap2"
which fails with a "Invalid cross-device link" error (EXDEV).
Fix this by adjusting the logic behind dataset traversal in
zfs_iter_children() to pick the right snapshot to send from.
Additionally update dry-run 'zfs send -t' to print its output to
stderr: this is consistent with other dry-run commands.

Patch Notes:
Reconciled differences between OpenZFS and
aee1dd4d983c64db3c3155290d48f05243e85709.

Authored by: loli10K <[email protected]>
Reviewed by: Paul Dagnelie <[email protected]>
Reviewed by: Matthew Ahrens <[email protected]>
Reviewed by: Brian Behlendorf <[email protected]>
Approved by: Hans Rosenfeld <[email protected]>
Ported-by: Giuseppe Di Natale <[email protected]>

OpenZFS-issue: https://www.illumos.org/issues/8940
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/9f7867c206
Closes #7171

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