MDEV-26441: Linux-dependent construct in SST scripts

SST scripts currently use Linux-specific construction
to create a temporary directory if the path prefix for
that directory is specified by the user. This does not
work with FreeBSD. This commit adds support for FreeBSD.

No separate test required.

scripts/wsrep_sst_mariabackup.sh

@@ -949,8 +949,10 @@ then
         tmpdir=$(parse_cnf "$encgroups" 'tmpdir')
         if [ -z "$tmpdir" ]; then
             xtmpdir="$(mktemp -d)"
-        else
+        elif [ "$OS" = 'Linux' ]; then
             xtmpdir=$(mktemp '-d' "--tmpdir=$tmpdir")
+        else
+            xtmpdir=$(TMPDIR="$tmpdir"; mktemp '-d')
         fi
 
         wsrep_log_info "Using '$xtmpdir' as mariabackup temporary directory"

scripts/wsrep_sst_rsync.sh

@@ -725,8 +725,10 @@ EOF
             tmpdir=$(parse_cnf '--mysqld|sst' 'tmpdir')
             if [ -z "$tmpdir" ]; then
                tmpfile="$(mktemp)"
-            else
+            elif [ "$OS" = 'Linux' ]; then
                tmpfile=$(mktemp "--tmpdir=$tmpdir")
+            else
+               tmpfile=$(TMPDIR="$tmpdir"; mktemp '-d')
             fi
 
             wsrep_log_info "Extracting binlog files:"

scripts/wsrep_sst_xtrabackup-v2.sh

@@ -973,8 +973,10 @@ then
         tmpdir=$(parse_cnf "$encgroups" 'tmpdir')
         if [ -z "$tmpdir" ]; then
             xtmpdir="$(mktemp -d)"
-        else
+        elif [ "$OS" = 'Linux' ]; then
             xtmpdir=$(mktemp '-d' "--tmpdir=$tmpdir")
+        else
+            xtmpdir=$(TMPDIR="$tmpdir"; mktemp '-d')
         fi
 
         wsrep_log_info "Using '$xtmpdir' as xtrabackup temporary directory"