Fixes a bug with seek in buffered IO.

A bug with the iobuf_lseek functions has been fixed and these functions
have been optimized so the underlying lseek syscall may be avoided in most cases.
However these functions are not used so this bug had no incidence on the
others utilities.
Adds the header defines in iobuf_stdout.

The header defines in iobuf_stdout were not defined.
Fixes a redefined macro.

Now that IOBUF_SIZE has moved in the iobuf header we cannot define it
within the asciify command. We changed the buffer size in IN_IOBUF_SIZE
instead.
Adds the fpipe command.

The fpipe command binds the standard input and output to respectively
the write and read parts of a file. This is useful when dealing with
block devices, especially tty. A double buffer is used to ensure that
read and write operations will not block as long as there is space left
in the buffer.