Add 'zfs send --saved' flag

This commit adds the --saved (-S) to the 'zfs send' command.
This flag allows a user to send a partially received dataset,
which can be useful when migrating a backup server to new
hardware. This flag is compatible with resumable receives, so
even if the saved send is interrupted, it can be resumed.
The flag does not require any user / kernel ABI changes or any
new feature flags in the send stream format.

Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Alek Pinchuk <apinchuk@datto.com>
Reviewed-by: Paul Zuchowski <pzuchowski@datto.com>
Reviewed-by: Christian Schwarz <me@cschwarz.com>
Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #9007

include/libzfs.h

@@ -677,6 +677,9 @@ typedef struct sendflags {
 
 	/* include snapshot holds in send stream */
 	boolean_t holds;
+
+	/* stream represents a partially received dataset */
+	boolean_t saved;
 } sendflags_t;
 
 typedef boolean_t (snapfilter_cb_t)(zfs_handle_t *, void *);
@@ -688,6 +691,7 @@ extern int zfs_send_one(zfs_handle_t *, const char *, int, sendflags_t *,
 extern int zfs_send_progress(zfs_handle_t *, int, uint64_t *, uint64_t *);
 extern int zfs_send_resume(libzfs_handle_t *, sendflags_t *, int outfd,
     const char *);
+extern int zfs_send_saved(zfs_handle_t *, sendflags_t *, int, const char *);
 extern nvlist_t *zfs_send_resume_token_to_nvlist(libzfs_handle_t *hdl,
     const char *token);