[OB] Add cleanup, source.c

[msnyder@sonic.net]

> There's no call to do_cleanups in this function, so it's quite hard to
> see if this cleanup will be run or discarded if error is not called.

When are they ever discarded?

My last understanding of the mechanism was that the cleanups
always get called, eventually.  Presumably when we return to
the command / event loop.

The only reason to call do_cleanups, in my understanding,
is if you want to force them to be done right now instead
of later.