Turn shellcheck into a normal make target. Fix new files it caught

This checks every file it checked (and a few more),
but explicitly instead of "if it works it works" best-effort
(which wasn't that good anyway)

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #10512
Closes #12101

config/Shellcheck.am

@@ -0,0 +1,8 @@
+.PHONY: shellcheck
+shellcheck: $(SCRIPTS) $(SHELLCHECKSCRIPTS)
+if HAVE_SHELLCHECK
+	[ -z "$(SCRIPTS)$(SHELLCHECKSCRIPTS)" ] && exit; shellcheck $$([ -n "$(SHELLCHECK_SHELL)" ] && echo "--shell=$(SHELLCHECK_SHELL)") --exclude=SC1090,SC1091$(SHELLCHECK_IGNORE) --format=gcc $(SCRIPTS) $(SHELLCHECKSCRIPTS)
+else
+	@[ -z "$(SCRIPTS)$(SHELLCHECKSCRIPTS)" ] && exit; echo "skipping shellcheck of" $(SCRIPTS) $(SHELLCHECKSCRIPTS) "because shellcheck is not installed"
+endif
+	@set -e; for dir in $(SHELLCHECKDIRS); do $(MAKE) -C $$dir shellcheck; done

config/always-shellcheck.m4

@@ -0,0 +1,7 @@
+dnl #
+dnl # Check if shellcheck is available.
+dnl #
+AC_DEFUN([ZFS_AC_CONFIG_ALWAYS_SHELLCHECK], [
+	AC_CHECK_PROG([SHELLCHECK], [shellcheck], [yes])
+	AM_CONDITIONAL([HAVE_SHELLCHECK], [test "x$SHELLCHECK" = "xyes"])
+])

config/zfs-build.m4

@@ -207,6 +207,7 @@ AC_DEFUN([ZFS_AC_CONFIG_ALWAYS], [
 	ZFS_AC_CONFIG_ALWAYS_PYZFS
 	ZFS_AC_CONFIG_ALWAYS_SED
 	ZFS_AC_CONFIG_ALWAYS_CPPCHECK
+	ZFS_AC_CONFIG_ALWAYS_SHELLCHECK
 ])
 
 AC_DEFUN([ZFS_AC_CONFIG], [