scripts: Fix ShellCheck diagnostics

Quote scalar arguments and use arrays for command option lists so paths
and options retain their boundaries. Preserve intentional version-field
splitting with narrow documented suppressions.

Also handle GNU getopt output without losing quoted arguments and
remove warnings about masked statuses, unused values and ambiguous test
expressions.

scripts/kernel-functions

@@ -84,9 +84,10 @@ function download_file {
 # Make sure the kernel tarball and patch file are present in directory
 # ${kernel_downloads}. Download any missing files from ${kernel_mirror}.
 function download_kernel {
-  local kver="$(kernel_version "$1")"
-  local plevel="$(patchlevel "$1")"
-  local series="$1"
+  local kver plevel series="$1"
+
+  kver="$(kernel_version "$1")"
+  plevel="$(patchlevel "$1")"
 
   series="${series/.*/}.x"
 
@@ -108,9 +109,10 @@ function download_kernel {
 }
 
 function extract_kernel_archive {
-  local kver="$(kernel_version "$1")"
-  local plevel="$(patchlevel "$1")"
-  local series="$1"
+  local kver plevel series="$1"
+
+  kver="$(kernel_version "$1")"
+  plevel="$(patchlevel "$1")"
 
   if [ -e "${kernel_downloads}/linux-$1.tar.xz" ]; then
     ( set -o pipefail
@@ -134,10 +136,12 @@ function extract_kernel_archive {
 # Create a linux-$1 tree in the current directory, where $1 is a kernel
 # version number with either three or four components.
 function extract_kernel_tree {
-  local kver="$(kernel_version "$1")"
-  local plevel="$(patchlevel "$1")"
+  local kver plevel
   local tmpdir=kernel-tree-tmp-$$
 
+  kver="$(kernel_version "$1")"
+  plevel="$(patchlevel "$1")"
+
   rm -rf "linux-$1" "${tmpdir}"
   mkdir "${tmpdir}" || return $?
   (
@@ -702,11 +706,9 @@ EOF
 
     # After patch-v4.14.1[12] has been applied, the execute bit has to be
     # set for sync-check.sh since patch can't do that.
-    for f in "tools/objtool/sync-check.sh"; do
-      if [ -e "$f" ]; then
-	chmod a+x "$f"
-      fi
-    done
+    if [ -e "tools/objtool/sync-check.sh" ]; then
+      chmod a+x "tools/objtool/sync-check.sh"
+    fi
 }
 
 function rpm_payload_is_readable {
@@ -770,7 +772,9 @@ function extract_rhel_kernel_archive {
 }
 
 function download_and_extract_distro_rpm {
-    [ -n "$1" ] || return $?
+    [ -n "$1" ] || return 1
+    # Split the kernel^distro^release triplet into positional parameters.
+    # shellcheck disable=SC2086
     set -- ${1//^/ }
     local kver=$1
     local distro=$2