ZTS: Provide an alternative to shuf for FreeBSD

There was a shuf package but the upstream for the port has recently
disappeared, so it is no longer available.

Create a function to hide the usage of shuf. Implement using seq|random
on FreeBSD.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9824

tests/zfs-tests/tests/functional/mv_files/random_creation.ksh

@@ -11,7 +11,7 @@ DIR="${TESTDIR}/RANDOM_SMALL"
 log_must mkdir "${DIR}"
 
 count=0
-for i in $(shuf -i 1-"${RC_PASS1}") ; do
+for i in $(range_shuffle 1 "${RC_PASS1}") ; do
     if ! touch "${DIR}/${i}" ; then
 	    log_fail "error creating ${i} after ${count} files"
     fi

tests/zfs-tests/tests/functional/zvol/zvol_swap/zvol_swap_005_pos.ksh

@@ -63,9 +63,9 @@ typeset -i pageblocks volblocks max_swaplow
 ((max_swaplow = (volblocks - (pageblocks * 2))))
 
 for i in {0..10}; do
-	swaplow=$(shuf -n 1 -i ${pageblocks}-${max_swaplow})
+	swaplow=$(range_shuffle ${pageblocks} ${max_swaplow} | head -n 1)
 	((maxlen = max_swaplow - swaplow))
-	swaplen=$(shuf -n 1 -i ${pageblocks}-${maxlen})
+	swaplen=$(range_shuffle ${pageblocks} ${maxlen} | head -n 1)
 	log_must swap -a $swapname $swaplow $swaplen
 	log_must swap -d $swapname $swaplow
 done