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/zfs/Makefile.am

@@ -1,5 +1,6 @@
+include $(top_srcdir)/config/Substfiles.am
+
 pkgsysconfdir = $(sysconfdir)/zfs
-initconfdir = $(DEFAULT_INITCONF_DIR)
 
 dist_pkgsysconf_DATA = \
 	vdev_id.conf.alias.example \
@@ -7,16 +8,8 @@ dist_pkgsysconf_DATA = \
 	vdev_id.conf.sas_switch.example \
 	vdev_id.conf.multipath.example \
 	vdev_id.conf.scsi.example
+
 pkgsysconf_SCRIPTS = \
 	zfs-functions
 
-EXTRA_DIST = zfs-functions.in
-
-$(pkgsysconf_SCRIPTS):%:%.in Makefile
-	  -$(SED) \
-		 -e 's,@sbindir\@,$(sbindir),g' \
-		 -e 's,@sysconfdir\@,$(sysconfdir),g' \
-		 -e 's,@initconfdir\@,$(initconfdir),g' \
-		 $< >'$@'
-
-CLEANFILES = $(pkgsysconf_SCRIPTS)
+SUBSTFILES += $(pkgsysconf_SCRIPTS)