Fix interrupted write in StreamCopyThread

If a flush() gets delivered at the same time that we are blocking
while writing to an interruptable stream, the copy thread will
abort assuming its a stream error.  Instead ignore the interrupt,
and retry the write.

Change-Id: Icbf62d1b8abe0fabbb532dbee088020eecf4c6c2
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>