Adding a new FOP

Steps to be followed when adding a new FOP to GlusterFS:

1. Edit glusterfs.h and add a GF_FOP_* constant.
2. Edit xlator.[ch] and:
   • add the new prototype for fop and callback.
   • edit xlator_fops structure.
3. Edit xlator.c and add to fill_defaults.
4. Edit protocol.h and add struct necessary for the new FOP.
5. Edit defaults.[ch] and provide default implementation.
6. Edit call-stub.[ch] and provide stub implementation.
7. Edit client-protocol and add your FOP.
8. Edit server-protocol and add your FOP.
9. Implement your FOP in any translator for which the default implementation is not sufficient.