Dynamic modules: do not overwrite old modules on install.

Just using "cp" is incorrect, as it will overwrite old files
possibly used by OS, leading to unexpected effects.  Changed
to "mv + cp", much like used for the main binary.
diff --git a/auto/install b/auto/install
index 6efa92f..948ddc1 100644
--- a/auto/install
+++ b/auto/install
@@ -169,17 +169,31 @@
 fi
 
 
-if test -n "$NGX_MODULES"; then
+if test -n "$DYNAMIC_MODULES"; then
     cat << END                                                >> $NGX_MAKEFILE
 
 	test -d '\$(DESTDIR)$NGX_MODULES_PATH' \
 		|| mkdir -p '\$(DESTDIR)$NGX_MODULES_PATH'
-	cp $NGX_MODULES '\$(DESTDIR)$NGX_MODULES_PATH'
 END
 
 fi
 
 
+for ngx_module in $DYNAMIC_MODULES
+do
+    ngx_module=$ngx_module$ngx_modext
+
+    cat << END                                                >> $NGX_MAKEFILE
+
+	test ! -f '\$(DESTDIR)$NGX_MODULES_PATH/$ngx_module' \
+		|| mv '\$(DESTDIR)$NGX_MODULES_PATH/$ngx_module' \
+			'\$(DESTDIR)$NGX_MODULES_PATH/$ngx_module.old'
+	cp $NGX_OBJS/$ngx_module '\$(DESTDIR)$NGX_MODULES_PATH/$ngx_module'
+END
+
+done
+
+
 # create Makefile
 
 cat << END >> Makefile