Centralize variable substitution

A bunch of places need to edit files to incorporate the configured paths
i.e. bindir, sbindir etc. Move this logic into a common file.

Create arc_summary by copying arc_summary[23] as appropriate at build
time instead of install time.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10559

etc/systemd/system/Makefile.am

@@ -1,3 +1,5 @@
+include $(top_srcdir)/config/Substfiles.am
+
 systemdpreset_DATA = \
 	50-zfs.preset
 
@@ -12,27 +14,8 @@ systemdunit_DATA = \
 	zfs-volumes.target \
 	zfs.target
 
-EXTRA_DIST = \
-	zfs-zed.service.in \
-	zfs-import-cache.service.in \
-	zfs-import-scan.service.in \
-	zfs-mount.service.in \
-	zfs-share.service.in \
-	zfs-import.target.in \
-	zfs-volume-wait.service.in \
-	zfs-volumes.target.in \
-	zfs.target.in \
-	50-zfs.preset.in
-
-$(systemdunit_DATA) $(systemdpreset_DATA):%:%.in
-	-$(SED) -e 's,@bindir\@,$(bindir),g' \
-		-e 's,@runstatedir\@,$(runstatedir),g' \
-		-e 's,@sbindir\@,$(sbindir),g' \
-		-e 's,@sysconfdir\@,$(sysconfdir),g' \
-		$< >'$@'
+SUBSTFILES += $(systemdpreset_DATA) $(systemdunit_DATA)
 
 install-data-hook:
 	$(MKDIR_P) "$(DESTDIR)$(systemdunitdir)"
 	ln -sf /dev/null "$(DESTDIR)$(systemdunitdir)/zfs-import.service"
-
-CLEANFILES = $(systemdunit_DATA) $(systemdpreset_DATA)