exec: add options -nf and -cl/-nocl in command /exec

-nf: create a new buffer with free content
-cl: clear new buffer
-nocl: append in new buffer

src/plugins/exec/exec-buffer.h

@@ -22,6 +22,8 @@
 
 extern void exec_buffer_set_callbacks ();
 extern struct t_gui_buffer *exec_buffer_new (const char *name,
+                                             int free_content,
+                                             int clear_buffer,
                                              int switch_to_buffer);
 
 #endif /* WEECHAT_EXEC_BUFFER_H */