support multi highlevel packagemanager.

HIGHLEVEL_PACKAGE_MANAGER was fordid in etckeeper.conf.
(apt, pacman-g2, yum, zypper etc)
This variable can not use multi highlevel packagemanager in 1 OS.
For example, openSUSE supports both ZYpp and YUM.
But HIGHLEVEL_PACKAGE_MANAGER can not write both package manager.
Instead of HIGHLEVEL_PACKAGE_MANAGER,
use ENABLE_APT=1, ENABLE_YUM=1, ENABLE_ZYPPER=1 !
eg.
ENABLE_APT=1
ENABLE_YUM=0
ENABLE_ZYPPER=0

Now, HIGHLEVEL_PACKAGE_MANAGER is an environment variable that etckeeper uses
inside.
Users should not define HIGHLEVEL_PACKAGE_MANAGER.

	modified:   Makefile
	modified:   apt.conf
	modified:   etckeeper
	modified:   etckeeper.conf
	modified:   pacman-g2.hook
	modified:   yum-etckeeper.py
	modified:   zypper-etckeeper.py