Add --enable-test-io-debug configure option which causes e2fsck and

Add --enable-test-io-debug configure option which causes e2fsck and
tune2fs to use the test I/O manager.

The test I/O manager has been changed to not do anything extra by
default, unless the TEST_IO_FLAGS and/or TEST_IO_BLOCK environment
variables are set, which controls what I/O operations are logged and
a block number to watch, respectively.  The log messages are sent to
stderr by default, unless a filename is specified via the
TEST_IO_LOGFILE environment variable.