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

scripts/zfs-tests.sh

@@ -301,10 +301,6 @@ constrain_path() {
 	elif [ "$UNAME" = "FreeBSD" ] ; then
 		ln -fs /usr/local/bin/ksh93 "$STF_PATH/ksh"
 	fi
-
-	if [ -L "$STF_PATH/arc_summary3" ]; then
-		ln -fs "$STF_PATH/arc_summary3" "$STF_PATH/arc_summary"
-	fi
 }
 
 #