Expose handle (file descriptor) from IoRead when applicable.

Some IO objects have file descriptors which can be useful for monitoring with select().

It might also be useful to expose handles for write objects but there is currently no use case.

test/src/module/common/ioTest.c

@@ -519,6 +519,7 @@ testRun(void)
                 MEM_CONTEXT_TEMP_END();
 
                 ioReadOpen(ioHandleReadIo(read));
+                TEST_RESULT_INT(ioReadHandle(ioHandleReadIo(read)), read->handle, "check handle");
 
                 // Read a string
                 TEST_RESULT_STR(strPtr(ioReadLine(ioHandleReadIo(read))), "test string 1", "read test string");