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/modules-load.d/Makefile.am

@@ -1,12 +1,2 @@
-modulesload_DATA = \
+dist_modulesload_DATA = \
 	zfs.conf
-
-EXTRA_DIST = \
-	zfs.conf.in
-
-$(modulesload_DATA):%:%.in
-	-$(SED) \
-		-e '' \
-		$< >'$@'
-
-CLEANFILES = $(modulesload_DATA)