script/check-config.sh: fix remaining shellcheck warnings

... and add this file to shellcheck target in Makefile.

These:

	In script/check-config.sh line 27:
	kernelMinor="${kernelVersion#$kernelMajor.}"
				     ^----------^ SC2295 (info): Expansions inside ${..} need to be quoted separately, otherwise they match as patterns.

	Did you mean:
	kernelMinor="${kernelVersion#"$kernelMajor".}"

	In script/check-config.sh line 103:
		source /etc/os-release 2>/dev/null || /bin/true
		       ^-------------^ SC1091 (info): Not following: /etc/os-release was not specified as input (see shellcheck -x).

	In script/check-config.sh line 267:
		NET_CLS_CGROUP $netprio
			       ^------^ SC2206 (warning): Quote to prevent word splitting/globbing, or split robustly with mapfile or read -a.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit d66498e771)
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Makefile

@@ -133,7 +133,7 @@ cfmt:
 shellcheck:
 	shellcheck tests/integration/*.bats tests/integration/*.sh \
 		tests/integration/*.bash tests/*.sh \
-		script/release_*.sh script/seccomp.sh script/lib.sh
+		script/*
 	# TODO: add shellcheck for more sh files
 
 shfmt:

script/check-config.sh

@@ -24,7 +24,7 @@ fi
 
 kernelVersion="$(uname -r)"
 kernelMajor="${kernelVersion%%.*}"
-kernelMinor="${kernelVersion#$kernelMajor.}"
+kernelMinor="${kernelVersion#"$kernelMajor".}"
 kernelMinor="${kernelMinor%%.*}"
 
 kernel_lt() {
@@ -113,7 +113,9 @@ check_flags() {
 }
 
 check_distro_userns() {
-	source /etc/os-release 2>/dev/null || /bin/true
+	[ -r /etc/os-release ] || return 0
+	# shellcheck source=/dev/null
+	. /etc/os-release 2>/dev/null || return 0
 	if [[ "${ID}" =~ ^(centos|rhel)$ && "${VERSION_ID}" =~ ^7 ]]; then
 		# this is a CentOS7 or RHEL7 system
 		grep -q "user_namespace.enable=1" /proc/cmdline || {
@@ -277,7 +279,7 @@ flags=(
 	BLK_CGROUP BLK_DEV_THROTTLING
 	CGROUP_PERF
 	CGROUP_HUGETLB
-	NET_CLS_CGROUP $netprio
+	NET_CLS_CGROUP "$netprio"
 	CFS_BANDWIDTH FAIR_GROUP_SCHED RT_GROUP_SCHED
 	IP_NF_TARGET_REDIRECT
 	IP_VS