Fix compilation problems in 'amp' plugin.

Four instances where 'default:' is at the end of a block replaced with
'default: break;' (reported by mscherer).

One typo where [j] should have been [1] (array out of bounds error).

Missing <unistd.h> includes for _exit(), and <string.h> for memcpy().

dll/amp/position.c

@@ -11,6 +11,8 @@
 #define POSITION
 #include "position.h"
 
+#include <string.h>
+
 /* Returns the number of frames actually skipped, -1 on error.
  *
  * Values in header are not changed if retval!=nframes.