Makefile.install: DESTDIR should be empty

DESTDIR is mostly used by packagers (such as from rpm spec
or deb rules), or just to try installation, and normally
should be left unset/empty.

Setting it to / looks weird to me. Besides, by leaving it empty
we are making sure everything works if DESTDIR is not set.

Fix INSTALL.md accordingly.

travis-ci: success for Makefile.install: DESTDIR should be empty
CC: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

INSTALL.md

@@ -8,7 +8,7 @@ and etc) simply typing
 
 this command accepts the following variables:
 
- * **DESTDIR**, to specify global root where all components will be placed under (`/` by default);
+ * **DESTDIR**, to specify global root where all components will be placed under (empty by default);
  * **PREFIX**, to specify additional prefix for path of every component installed (`/usr/local` by default);
  * **BINDIR**, to specify where to put CRIT tool (`$(PREFIX)/bin` by default);
  * **SBINDIR**, to specify where to put CRIU executable (`$(PREFIX)/sbin` by default);

Makefile.install

 #
 # Installation paths.
-DESTDIR		?= /
 PREFIX		?= /usr/local
 BINDIR		?= $(PREFIX)/bin
 SBINDIR		?= $(PREFIX)/sbin