add -Wall to CFALGS

Though maintainer.mak, most of all contributors may use Makefile. So
they may not have a change to run gcc with -Wall. With this patch
they will have the chance before sending pull-request.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
remove an unused variable

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
prepare the way to reset data-path

A user can make data-path empty with --data-path=NONE in command line.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>