Use 'Returns:' instead of 'Return:' in functions' docs

This is the right keyword for documenting the returned value.
Provide stdout/stderr output if mkswap's status != 0

It carries important information for the caller in such cases.

Also add basic test for the bd_swap_mkswap function.
Hook up the swap plugin and add it to the make targets
Free the argv array as soon as it becomes useless

It should be free'd in any case.