Address errors reported by CLANG

_entity_add_data() in src/common/entity.c defined as "static" to avoid
    confusion with same function name in src/common/layouts_mgr.c
_entity_get_node_walkfunc() in src/common/entity.c defined as "static"
Removed redundant set of "rc" variable _layouts_autoupdate_layout_if_allowed(),
    src/common/layouts_mgr.c
Initialized variable "rc" in _layouts_read_state(), src/common/layouts_mgr.c
    in case the named file does not exist (stat() returns an error)
The CLANG tool reported a possible error in _autoupdate_layout_tree(),
    src/common/layouts_mgr.c in the event that xtree_node_get_data()
    returns a NULL value and that pointer is used later in the function for
    "cnode->entity". I added "return 1" if xtree_node_get_data() returns NULL,
    but am not yet sure if that is the correct action.
The CLANG tool reported possible errors related to a NULL value pointer in
    s_p_parse_pair_with_op(), src/common/parse_config.c which have been fixed.