Correct shellcheck errors

The ZFS buildbot moved to using Ubuntu 17.04 for the
STYLE builder which has a newer version of shellcheck.
Correct the new issues it discovers.

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #6647

scripts/zfs-helpers.sh

@@ -88,11 +88,11 @@ while getopts 'hdirv' OPTION; do
 	esac
 done
 
-if [ "$INSTALL" = "yes" -a "$REMOVE" = "yes" ]; then
+if [ "$INSTALL" = "yes" ] && [ "$REMOVE" = "yes" ]; then
 	fail "Specify -i or -r but not both"
 fi
 
-if [ "$INSTALL" = "no" -a "$REMOVE" = "no" ]; then
+if [ "$INSTALL" = "no" ] && [ "$REMOVE" = "no" ]; then
 	fail "Either -i or -r must be specified"
 fi
 
@@ -115,8 +115,8 @@ if [ "$VERBOSE" = "yes" ]; then
 fi
 
 install() {
-	local src=$1
-	local dst=$2
+	src=$1
+	dst=$2
 
 	if [ -h "$dst" ]; then
 		echo "Symlink exists: $dst"
@@ -136,7 +136,7 @@ install() {
 }
 
 remove() {
-	local dst=$1
+	dst=$1
 
 	if [ -h "$dst" ]; then
 		msg "rm $dst"