Makefiles: add install target

Note to package maintainers:

1 Yes we do support DESTDIR.

2 From packaging scripts, use something like this:

	make install DESTDIR=$RPM_BUILD_ROOT PREFIX=/usr

(v2: add PREFIX, install to /usr/local not /usr by default)
(v3: fix criu man section -- it is 8 not 1)

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>