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

cmd/dbufstat/.gitignore

@@ -0,0 +1 @@
+dbufstat

cmd/dbufstat/Makefile.am

@@ -1,13 +1,5 @@
-dist_bin_SCRIPTS = dbufstat
+include $(top_srcdir)/config/Substfiles.am
 
-#
-# The dbufstat script is compatible with both Python 2.6 and 3.4.
-# As such the python 3 shebang can be replaced at install time when
-# targeting a python 2 system.  This allows us to maintain a single
-# version of the source.
-#
-if USING_PYTHON_2
-install-exec-hook:
-	$(SED) ${ac_inplace} -e 's|^#!/usr/bin/env python3|#!/usr/bin/env python2|' \
-	    $(DESTDIR)$(bindir)/dbufstat
-endif
+bin_SCRIPTS = dbufstat
+
+SUBSTFILES += $(bin_SCRIPTS)

cmd/dbufstat/dbufstat.in

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env @PYTHON_SHEBANG@
 #
 # Print out statistics for all cached dmu buffers.  This information
 # is available through the dbufs kstat and may be post-processed as