channel: Set sockets as nonblocking to prevent deadlocks when copying

The channel socket pair is not set as nonblocking currently, which may cause
deadlocks in some cases (e.g. in-mount copy over read-write fallback), because
g_output_stream_write_async may block. This issue appears after increasing max
size of buffer in read channel:
https://bugzilla.gnome.org/show_bug.cgi?id=773826

Set channel sockets as nonblocking to be sure that _async methods don't block.

https://bugzilla.gnome.org/show_bug.cgi?id=785391