* dcrt0.cc: Add load statements for `GetSidIdentifierAuthority'

    * dcrt0.cc: Add load statements for `GetSidIdentifierAuthority'
        and `RegLoadKeyA'.
        * registry.cc (get_registry_hive_path): New function.
        (load_registry_hive): Ditto.
        * security.cc (convert_sid_to_string_sid): New function.
        (get_ssid): Renamed to `convert_string_sid_to_sid'.
        (get_pw_sid): Call `convert_string_sid_to_sid' instead of `get_ssid'.
        (get_gr_sid): Ditto.
        (get_admin_sid): Ditto.
        (get_system_sid): Ditto.
        (get_creator_owner_sid): Ditto.
        (get_world_sid): Ditto.
        * shared.h: New prototypes for `get_registry_hive_path' and
        `load_registry_hive'.
        * spawn.cc (spawn_guts): Set child->psid to NULL to force calling
        `internal_getlogin' from child process in case of changing user context.
        Call `load_registry_hive' in case of changing user context.
        (_spawnve): Copy user infos only if user context remains the same.
        * uinfo.cc: Add load statement for `NetUserGetInfo'.
        Remove load statement for `NetGetDCName'.
        (internal_getlogin): Rewrite to speed up process startup
        and to correct user environment in case user context changes.
        (uinfo_init): Call internal_getlogin only if myself->psid is NULL,
        that is user context changes.
        * winsup.h: Add prototypes for `convert_sid_to_string_sid',
        `convert_string_sid_to_sid' and `get_pw_sid'.