libext2fs: support atexit cleanups

libext2fs: support atexit cleanups

Use the atexit() function to provide a means for the library to clean
itself up on program exit.  This will be used by the undo IO manager
to flush the undo file state to disk if the program should terminate
without closing the io channel, since most e2fsprogs clients will
simply exit() when they hit errors.

This won't help for signal termination; client programs must set
up signal handlers.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>