Don't close stream twice when splicing

Don't close stream twice when splicing

Ensure that the output/target stream in a g_output_stream_splice_async()
operation is marked as closed if G_OUTPUT_STREAM_SPLICE_CLOSE_TARGET is
passed to g_output_stream_splice_async(). This removes the possibility of
local FDs being closed twice because the stream's not marked as closed.

This is implemented by calling g_output_stream_close() from within
g_output_stream_splice_async() instead of calling the stream's close_fn()
directly.

Closes: bgo#659324