Restore :: in Makefile.am

The double-colon looked like a typo, but it's actually an obscure
feature. Rules with :: may appear multiple times and are run
independently of one another in the order they appear. The use of ::
for distclean-local was conventional, not accidental.

Add comments to indicate the intentional use of double-colon rules.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9210

etc/init.d/Makefile.am

@@ -40,5 +40,6 @@ $(init_SCRIPTS) $(initconf_SCRIPTS) $(initcommon_SCRIPTS):%:%.in
 	  [ '$@' = 'zfs-functions' -o '$@' = 'zfs' ] || \
 		chmod +x '$@')
 
+# Double-colon rules are allowed; there are multiple independent definitions.
 distclean-local::
 	-$(RM) $(init_SCRIPTS) $(initcommon_SCRIPTS) $(initconf_SCRIPTS)

etc/modules-load.d/Makefile.am

@@ -9,5 +9,6 @@ $(modulesload_DATA):%:%.in
 		-e '' \
 		$< >'$@'
 
+# Double-colon rules are allowed; there are multiple independent definitions.
 distclean-local::
 	-$(RM) $(modulesload_DATA)

etc/systemd/system-generators/Makefile.am

@@ -11,5 +11,6 @@ $(systemdgenerator_SCRIPTS): %: %.in
 		-e 's,@sysconfdir\@,$(sysconfdir),g' \
 		$< >'$@'
 
+# Double-colon rules are allowed; there are multiple independent definitions.
 distclean-local::
 	-$(RM) $(systemdgenerator_SCRIPTS)

etc/systemd/system/Makefile.am

@@ -35,5 +35,6 @@ install-data-hook:
 	$(MKDIR_P) "$(DESTDIR)$(systemdunitdir)"
 	ln -sf /dev/null "$(DESTDIR)$(systemdunitdir)/zfs-import.service"
 
+# Double-colon rules are allowed; there are multiple independent definitions.
 distclean-local::
 	-$(RM) $(systemdunit_DATA) $(systemdpreset_DATA)