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/init.d/Makefile.am

@@ -1,38 +1,7 @@
-initdir = $(DEFAULT_INIT_DIR)
+include $(top_srcdir)/config/Substfiles.am
+
+EXTRA_DIST += README.md
+
 init_SCRIPTS = zfs-import zfs-mount zfs-share zfs-zed
 
-initconfdir = $(DEFAULT_INITCONF_DIR)
-
-EXTRA_DIST = \
-	zfs-share.in \
-	zfs-import.in \
-	zfs-mount.in \
-	zfs-zed.in \
-	README.md
-
-$(init_SCRIPTS):%:%.in Makefile
-	-(if [ -e /etc/debian_version ]; then \
-		NFS_SRV=nfs-kernel-server; \
-	  else \
-		NFS_SRV=nfs; \
-	  fi; \
-	  if [ -e /sbin/openrc-run ]; then \
-		SHELL=/sbin/openrc-run; \
-	  else \
-		SHELL=/bin/sh; \
-	  fi; \
-	  $(SED) \
-		 -e 's,@bindir\@,$(bindir),g' \
-		 -e 's,@sbindir\@,$(sbindir),g' \
-		 -e 's,@udevdir\@,$(udevdir),g' \
-		 -e 's,@udevruledir\@,$(udevruledir),g' \
-		 -e 's,@sysconfdir\@,$(sysconfdir),g' \
-		 -e 's,@initconfdir\@,$(initconfdir),g' \
-		 -e 's,@initdir\@,$(initdir),g' \
-		 -e 's,@runstatedir\@,$(runstatedir),g' \
-		 -e "s,@SHELL\@,$$SHELL,g" \
-		 -e "s,@NFS_SRV\@,$$NFS_SRV,g" \
-		 $< >'$@'; \
-		chmod +x '$@')
-
-CLEANFILES = $(init_SCRIPTS)
+SUBSTFILES += $(init_SCRIPTS)