grn_column_name: return fully qualified name for accessor
grn_column_name: return fully qualified name for accessor

Before:

    grn_table_select() ->
      grn_obj_column("column1.column2") ->
      # _key.column1.column2 accessor
      grn_column_name() ->
      "_key"

After:

    grn_table_select() ->
      grn_obj_column("column1.column2") ->
      # _key.column1.column2 accessor
      grn_column_name() ->
      "column1.column2"
    # internal "_key" is omitted.

naoa/groonga,cosmo0920/groonga,cosmo0920/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,komainu8/groonga,komainu8/groonga,cosmo0920/groonga,naoa/groonga,komainu8/groonga,cosmo0920/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,kenhys/groonga,kenhys/groonga,naoa/groonga,komainu8/groonga,kenhys/groonga,cosmo0920/groonga,groonga/groonga,komainu8/groonga,naoa/groonga,groonga/groonga,naoa/groonga,cosmo0920/groonga,cosmo0920/groonga,kenhys/groonga,groonga/groonga,cosmo0920/groonga,groonga/groonga,kenhys/groonga,naoa/groonga,komainu8/groonga,groonga/groonga,kenhys/groonga,naoa/groonga,naoa/groonga,komainu8/groonga