Prefer ?* to hide passwords

Prefer ?* to hide passwords

It might be argued that a hidden string is more legible when using
?* as the hidden character instead of ?.
For example, the following strings have the same length:
"......"

"******"

It's slighly easier to visually count the number of characters in the second
string (Bug#32220).

* lisp/subr.el (read-passwd): Prefer ?* as default char instead of ?.
* doc/lispref/minibuf.texi (Reading a Password): Update manual.
; * etc/NEWS (Changes in Emacs 27.1): Announce the change.