Correct man page dates

Various changes (many by me) have been made to the man pages without
bumping their dates.  I have now corrected them based on the last commit
to each file.  I also added the script I used to make these changes.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #8710

man/man8/zdb.8

@@ -15,7 +15,7 @@
 .\" Copyright (c) 2017 Lawrence Livermore National Security, LLC.
 .\" Copyright (c) 2017 Intel Corporation.
 .\"
-.Dd April 14, 2017
+.Dd April 14, 2019
 .Dt ZDB 8 SMM
 .Os Linux
 .Sh NAME

man/man8/zfs-program.8

@@ -10,7 +10,7 @@
 .\"
 .\" Copyright (c) 2016, 2019 by Delphix. All Rights Reserved.
 .\"
-.Dd January 21, 2016
+.Dd February 26, 2019
 .Dt ZFS-PROGRAM 8
 .Os
 .Sh NAME

man/man8/zfs.8

@@ -30,7 +30,7 @@
 .\" Copyright 2018 Nexenta Systems, Inc.
 .\" Copyright 2018 Joyent, Inc.
 .\"
-.Dd February 26, 2019
+.Dd April 30, 2019
 .Dt ZFS 8 SMM
 .Os Linux
 .Sh NAME

man/man8/zgenhostid.8

@@ -21,7 +21,7 @@
 .\"
 .\" Copyright (c) 2017 by Lawrence Livermore National Security, LLC.
 .\"
-.Dd July 24, 2017
+.Dd September 16, 2017
 .Dt ZGENHOSTID 8 SMM
 .Os Linux
 .Sh NAME

man/man8/zpool.8

@@ -27,7 +27,7 @@
 .\" Copyright 2017 Nexenta Systems, Inc.
 .\" Copyright (c) 2017 Open-E, Inc. All Rights Reserved.
 .\"
-.Dd November 29, 2018
+.Dd May 2, 2019
 .Dt ZPOOL 8 SMM
 .Os Linux
 .Sh NAME