Fix #867, better error translation for ESPIPE errno

The ESPIPE errno means that seeking is not supported on the
given file handle.  Translate to the OS_ERR_OPERATION_NOT_SUPPORTED
error rather than not implemented as it better indicates the
actual condition.

src/os/portable/os-impl-posix-io.c

@@ -138,7 +138,7 @@ int32 OS_GenericSeek_Impl(const OS_object_token_t *token, int32 offset, uint32 w
              * Use a different error code to differentiate from an
              * error involving a bad whence/offset
              */
-            retval = OS_ERR_NOT_IMPLEMENTED;
+            retval = OS_ERR_OPERATION_NOT_SUPPORTED;
         }
         else
         {

src/unit-test-coverage/portable/src/coveragetest-posix-io.c

@@ -84,7 +84,7 @@ void Test_OS_GenericSeek_Impl(void)
 
     /* The seek implementation also checks for this specific pipe errno */
     OCS_errno = OCS_ESPIPE;
-    OSAPI_TEST_FUNCTION_RC(OS_GenericSeek_Impl, (&token, 0, OS_SEEK_END), OS_ERR_NOT_IMPLEMENTED);
+    OSAPI_TEST_FUNCTION_RC(OS_GenericSeek_Impl, (&token, 0, OS_SEEK_END), OS_ERR_OPERATION_NOT_SUPPORTED);
 }
 
 void Test_OS_GenericRead_Impl(void)