Implement ECMA-119 burn_source and writer thread handling.

At this time, nglibisofs is able to output an image!! Note that communication
between writer and read thread is done via a pipe. That will be replaced by a
ring buffer in the near future.

src/error.h

@@ -22,6 +22,7 @@
 #define ISO_WRONG_ARG_VALUE             -6
 
 #define ISO_WRITE_ERROR                 -10
+#define ISO_THREAD_ERROR                -11
 
 #define ISO_NODE_ALREADY_ADDED          -50
 #define ISO_NODE_NAME_NOT_UNIQUE        -51