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

man/man8/Makefile.am

@@ -1,3 +1,5 @@
+include $(top_srcdir)/config/Substfiles.am
+
 dist_man_MANS = \
 	fsck.zfs.8 \
 	mount.zfs.8 \
@@ -86,16 +88,4 @@ nodist_man_MANS = \
 	zed.8 \
 	zfs-mount-generator.8
 
-EXTRA_DIST = \
-	zed.8.in \
-	zfs-mount-generator.8.in
-
-$(nodist_man_MANS): %: %.in
-	-$(SED) -e 's,@zfsexecdir\@,$(zfsexecdir),g' \
-		-e 's,@systemdgeneratordir\@,$(systemdgeneratordir),g' \
-		-e 's,@runstatedir\@,$(runstatedir),g' \
-		-e 's,@sysconfdir\@,$(sysconfdir),g' \
-		$< >'$@'
-
-CLEANFILES = \
-	$(nodist_man_MANS)
+SUBSTFILES += $(nodist_man_MANS)