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
GOutputStream: Add g_output_stream_async_write_is_via_threads()

In implementing a better g_output_stream_splice_async() and possibly
other situtations it's helpful to know whether the output stream's
write function internally uses threads. If it and the input stream's
read 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 output stream's
g_output_stream_write_async() function internally uses threads.

https://bugzilla.gnome.org/show_bug.cgi?id=691581
tests: Add testcases for g_output_stream_splice_async()

Previously, no testcases tested the close flags of
g_output_stream_splice_async. This patch adds tests for that and
also tests various combinations of threaded and non-threaded
GInputStream async reads and GOutputStream async writes.

https://bugzilla.gnome.org/show_bug.cgi?id=691581
GOutputStream: Rename _g_output_stream_close_internal() for consistency

https://bugzilla.gnome.org/show_bug.cgi?id=691581
GOutputStream: Split _close_async for internal use

Refactor g_output_stream_close_async() into itself and an internal
variant for potential use inside other operations (splice_async).
The internal version must be called between
g_output_stream_set_pending() and g_output_stream_clear_pending().

https://bugzilla.gnome.org/show_bug.cgi?id=691581
GOutputStream: Use async read/write of streams in splice_async()

There are some corner cases where using the sync version of read/write
in a thread could cause thread-safety issues. In these cases it's
possible to override the output stream's splice_async() function,
but for input streams one would need to do some acrobatics to
stay thread-safe. Alternatively, some implementations may not even
override their sync read/write functions.

This patch refactors the default splice_async() implementation to
call the sync read and write functions in a thread only when both
async versions are thread-based. When one or both are non-threaded,
it calls the virtual write_async() and read_async() functions of the
involved streams within the same thread.

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