socket: cleanup use of ERRNO_IS_DISCONNECT()

socket: cleanup use of ERRNO_IS_DISCONNECT()

Given that ERRNO_IS_DISCONNECT() also matches positive values,
make sure this macro is not called with arguments that do not have
errno semantics.

In this case the arguments passed to ERRNO_IS_DISCONNECT() are the
values returned by instance_from_socket(), socket_load_service_unit(),
and service_set_socket_fd() which are not expected to return any
positive values, but let's be consistent anyway and move
ERRNO_IS_DISCONNECT() invocations to the branches where the return
values are known to be negative.