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
24 files changed