libext2fs: add threading support to the I/O manager abstraction

libext2fs: add threading support to the I/O manager abstraction

Add initial implementation support for the unix_io manager.
Applications which want to use threading should pass in
IO_FLAG_THREADS when opening the channel.  Channels which support
threading (which as of this commit is unix_io and test_io if the
backing io_manager supports threading) will set the
CHANNEL_FLAGS_THREADS bit in io->flags.  Library code or applications
can test if threading is enabled by checking this flag.

Applications using libext2fs can pass in EXT2_FLAG_THREADS to
ext2fs_open() or ext2fs_open2() to request threading support.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>