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

cmd/Makefile.am

@@ -1,3 +1,5 @@
+include $(top_srcdir)/config/Shellcheck.am
+
 SUBDIRS  = zfs zpool zdb zhack zinject zstream ztest
 SUBDIRS += fsck_zfs vdev_id raidz_test zfs_ids_to_path
 SUBDIRS += zpool_influxdb
@@ -5,6 +7,9 @@ SUBDIRS += zpool_influxdb
 CPPCHECKDIRS  = zfs zpool zdb zhack zinject zstream ztest
 CPPCHECKDIRS += raidz_test zfs_ids_to_path zpool_influxdb
 
+# TODO: #12084: SHELLCHECKDIRS = fsck_zfs vdev_id zpool
+SHELLCHECKDIRS = fsck_zfs zpool
+
 if USING_PYTHON
 SUBDIRS += arcstat arc_summary dbufstat
 endif
@@ -12,6 +17,7 @@ endif
 if BUILD_LINUX
 SUBDIRS += mount_zfs zed zgenhostid zvol_id zvol_wait
 CPPCHECKDIRS += mount_zfs zed zgenhostid zvol_id
+SHELLCHECKDIRS += zed
 endif
 
 PHONY = cppcheck