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

scripts/man-dates.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+
+# This script updates the date lines in the man pages to the date of the last
+# commit to that file.
+
+set -eu
+
+find man -type f | while read -r i ; do
+    git_date=$(git log -1 --date=short --format="%ad" -- "$i")
+    [ "x$git_date" = "x" ] && continue
+    sed -i "s|^\.Dd.*|.Dd $(date -d "$git_date" "+%B %-d, %Y")|" "$i"
+done