Support stacking multiple FSALs

* For a config file such as
EXPORT { FSAL { name = "stk1";
                FSAL { name = "stk2";
                       FSAL { name = "FS";
                            }
                     }
              }
       }
* Config parser processes EXPORT block and identifies FSAL associated with the
  export - 'stk1'. Library (libfsalstk1.so) is loaded and 'stk1' is registered
  as an FSAL.
* 'stk1_create_export' being a stackable FSAL is concerned about sub-FSAL right
  below it:
  FSAL { name = "stk1";
         FSAL { name = "stk2";
                ...
              }
       }
  After parsing the sub-FSAL name, subfsal_commit loads the sub-FSAL and gets
  it's 'create_export' function registered. 'stk2_create_export' is then called.
* 'stk2_create_export' behaves similar to 'stk1_create_export' - loads library
  'libfsalfs.so' and calls 'fs_create_export'
This enables stacking of as many stackable FSALs on top of a regular FSAL.

Change-Id: I9ed9373c63eb9d09b97d03da5ccc302f4df1819a
Signed-off-by: Arun Olappamanna Vasudevan <aolappamanna@cs.stonybrook.edu>
Reviewed-by: Ming Chen <mchen@cs.stonybrook.edu>