GInputStream: Add g_input_stream_async_read_is_via_threads()

GInputStream: Add g_input_stream_async_read_is_via_threads()

In implementing a better g_output_stream_splice_async() and possibly
other situtations it's helpful to know whether the input stream's
read function internally uses threads. If it and the output stream's
write async functions use threads, then the splice function could
spawn a single thread for better efficiency.

This patch adds a function to determine whether an input stream's
g_input_stream_read_async() function internally uses threads.

https://bugzilla.gnome.org/show_bug.cgi?id=691581