Move reading an extended command to Elisp (bug#5364, bug#5214).

Move reading an extended command to Elisp (bug#5364, bug#5214).

* lisp/simple.el (read-extended-command): New function with the logic
for `completing-read' moved to Elisp from `execute-extended-command'.
Use `function-called-at-point' in `minibuffer-default-add-function'
to get a command name for M-n (bug#5364, bug#5214).

* src/keyboard.c (Fexecute_extended_command): Move reading a command name
with `completing-read' to a new Elisp function `read-extended-command'.
Call it to read a command to `function'  (bug#5364, bug#5214).