Refactor sparse file support into libsparse

Minimal refactoring of output_file.c into libsparse in preparation
for completely separating libsparse from ext4_utils.

Moves output_file.c, backed_block.c, and parts of ext4_utils.c
into libsparse.  The only changes to the remanining files in
ext4_utils are using the new sparse.h header, and moving the
wipe call out of output_file.c and into make_ext4fs.c.

Change-Id: I1f66f6c3e05230a350023c5b4ea4422f16a73c4b
diff --git a/ext4_utils/ext4fixup.c b/ext4_utils/ext4fixup.c
index c664ac3..f0124f8 100644
--- a/ext4_utils/ext4fixup.c
+++ b/ext4_utils/ext4fixup.c
@@ -16,11 +16,11 @@
 #include "ext4_utils.h"
 #include "make_ext4fs.h"
 #include "ext4_extents.h"
-#include "output_file.h"
-#include "backed_block.h"
 #include "allocate.h"
 #include "ext4fixup.h"
 
+#include <sparse/sparse.h>
+
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/types.h>