Allow minibuffer default to be a list of default values.

Allow minibuffer default to be a list of default values.
With empty input use the first element of this list as returned default.
(string_to_object):
(read_minibuf_noninteractive):  If defalt is cons, set val to its car.
(read_minibuf): If defalt is cons, set histstring to its car.
(Fread_string): If default_value is cons, set val to its car.
(Fread_buffer): If def is cons, use its car.
(Fcompleting_read): If defalt is cons, set val to its car.