add some 'contributed' example programs.
io-interface change: open gets passed metadata.

stack will need this so it can identify which image a read should
go to.
ookbrick now reports its error code.

instead of implicitly setting errno (eww).
add partial implementation of stack io-interface.

The stack io-interface is an io-interface which abstracts over a
stack of images.
ignore contrib binaries.
Report io-interface's error when open fails.
ookmaxbricksize: return EINVAL when given a NULL ptr.
ookbrick/bricksize: make ookfile arg constant.
ookbrick/ookwrite: detect+report invalid arguments.
start on some man pages.