Refactor IO, add buffered IO

I wasn't happy with existing implementation of the IO, so I decided
to change it - as a result there is no longer BH_IOOpen and BH_IOClose
and many IO operations are now optional (behind BH_IOCtl).

Finnally implemented buffered IO and fixed size memory buffer IO.

include/BH/Common.h

@@ -12,6 +12,9 @@
 #define BH_FOUND        0x0005
 #define BH_NOTFOUND     0x0006
 #define BH_TIMEOUT      0x0007
+#define BH_SHORT        0x0008
+#define BH_FULL         0x0009
+
 
 #define BH_UNUSED(x)    (void)(x)
 #define BH_PTR2INT(x)   ((intptr_t)(x))