lib: iostream: Record the ioloop that the iostream was last switched to.

lib: When istream is destroyed, it now always unrefs the parent istream if it exists. This way the implementations don't have to do it themselves in their destroy() function. This change doesn't necessarily require changing the existing code, because if the istream implementation already unrefs the parent it gets set to NULL so the auto-unref won't be done.

lib: Added io_stream_get_disconnect_reason() to iostream.h

lib: Added o_stream_add_destroy_callback()

lib: Added [io]_stream_create_error_str()

iostream: Added ability to set/get error strings for streams.

istream API change: Added support for multiple destroy callbacks.

iostreams: Added close_parent flag to close() handler and clarified close/destroy APIs. This makes it unambiguous how things work: Unless you explicitly call [io]_stream_close(), the parent streams won't be closed. This is what most (hopefully all!) of the existing code expects. I was wondering a bit if [io]_stream_destroy() should simply have been removed and replaced with [io]_stream_unref() calls, since they would have worked basically everywhere, but there might be some places where it's better to have explicitly closed the stream (and where closing the parent stream doesn't matter).

Renamed lib/*-internal.h files to lib/*-private.h for consistency.