cfdisk: add libfdisk independent debug stuff

Now it's possible to use
	CFDISK_DEBUG=0xffff ./cfdisk
to debug only cfdisk UI.

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: clean up API (context.c)

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: clean up API (alignment.c)

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: cleanup fdisk_is_label stuff

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: cleanup label geometry API

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: make it possible to get fields for all labes

Signed-off-by: Karel Zak <kzak@redhat.com>