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

tests/zfs-tests/include/Makefile.am

@@ -1,3 +1,5 @@
+include $(top_srcdir)/config/Substfiles.am
+
 pkgdatadir = $(datadir)/@PACKAGE@/zfs-tests/include
 dist_pkgdata_DATA = \
 	blkdev.shlib \
@@ -8,13 +10,5 @@ dist_pkgdata_DATA = \
 	tunables.cfg \
 	zpool_script.shlib
 
-EXTRA_DIST = default.cfg.in
-
 nodist_pkgdata_DATA = default.cfg
-
-$(nodist_pkgdata_DATA): %: %.in
-	-$(SED) -e 's,@zfsexecdir\@,$(zfsexecdir),g' \
-		-e 's,@sysconfdir\@,$(sysconfdir),g' \
-		$< >'$@'
-
-CLEANFILES = default.cfg
+SUBSTFILES += $(nodist_pkgdata_DATA)

tests/zfs-tests/include/commands.cfg

@@ -178,7 +178,6 @@ export ZFS_FILES='zdb
     ztest
     raidz_test
     arc_summary
-    arc_summary3
     arcstat
     dbufstat
     zed

tests/zfs-tests/tests/functional/pyzfs/Makefile.am

@@ -1,17 +1,7 @@
+include $(top_srcdir)/config/Substfiles.am
+
 pkgpyzfsdir = $(datadir)/@PACKAGE@/zfs-tests/tests/functional/pyzfs
 pkgpyzfs_SCRIPTS = \
 	pyzfs_unittest.ksh
 
-EXTRA_DIST = \
-	pyzfs_unittest.ksh.in
-
-#
-# The pyzfs module is built either for Python 2 or Python 3.  In order
-# to properly test it the unit tests must be updated to the matching version.
-#
-$(pkgpyzfs_SCRIPTS):%:%.in
-	-$(SED) -e 's,@PYTHON\@,$(PYTHON),g' \
-		$< >'$@'
-	-chmod 775 $@
-
-CLEANFILES = $(pkgpyzfs_SCRIPTS)
+SUBSTFILES += $(pkgpyzfs_SCRIPTS)