Add `is_blocking()` method to FileHandle

There was no way to check current blocking state, so no way to modify
and restore status.

Also have default FileHandle::set_blocking() used by real files return a
correct error code when asked for non-blocking, and success when asked
for blocking.

These were minor omissions that are required to implement POSIX fcntl
properly.

fixup! Add `is_blocking()` method to FileHandle

drivers/UARTSerial.h

@@ -142,6 +142,15 @@ public:
         return 0;
     }
 
+    /** Check current blocking or non-blocking mode for file operations.
+     *
+     *  @return             true for blocking mode, false for non-blocking mode.
+     */
+    virtual bool is_blocking() const
+    {
+        return _blocking;
+    }
+
     /** Register a callback on state change of the file.
      *
      *  The specified callback will be called on state changes such as when

platform/FileHandle.h

@@ -192,7 +192,16 @@ public:
      */
     virtual int set_blocking(bool blocking)
     {
-        return -1;
+        return blocking ? 0 : -ENOTTY;
+    }
+
+    /** Check current blocking or non-blocking mode for file operations.
+     *
+     *  @return             true for blocking mode, false for non-blocking mode.
+     */
+    virtual bool is_blocking() const
+    {
+        return true;
     }
 
     /** Check for poll event flags