tests: loaders: Add IOWBuffer test.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: IO: Two fixes.

* Fix IOWBuffer close().

* Set errno on invalid whence.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: loaders; Update I/O documentation.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>