keyfile: various refactoring and restructure nm_keyfile_read()

- in nm_keyfile_read(), unify _read_setting() and
  _read_setting_vpn_secret() in they way they are called
  (that is, they no longer return any value and don't accept
  any arguments aside @info).

- use cleanup attributes

- use nm_streq() instead of strcmp().

- wrap lines that have multiple statements or conditions.

shared/nm-utils/nm-shared-utils.c

@@ -1196,7 +1196,7 @@ nm_g_object_set_property_boolean (GObject *object,
 
 gboolean
 nm_g_object_set_property_uint (GObject *object,
-                               const char   *property_name,
+                               const char *property_name,
                                guint value,
                                GError **error)
 {