(Fread_command): New optional arg DEFAULT-VALUE. Callers changed.

(Fread_command): New optional arg DEFAULT-VALUE.  Callers changed.
(Fread_variable, Fread_string, Fread_from_minibuffer): Likewise.
(read_minibuf): Now static.  New arg DEFALT.  Callers changed.
Bind minibuffer-default here.
(read_minibuf_unwind): Now static.
(Fcompleting_read): Past DEF to Fread_from_minibuffer;
no need to use it directly here.

(Fcompleting_read): New arg DEF.  Callers changed.
(Qminibuffer_default): New variable.
(syms_of_minibuf): Initialize it.