This function reads a password, prompting with prompt. It does
not echo the password as the user types it; instead, it echoes `.'
for each character in the password.
The optional argument confirm, if non-nil, says to read the
password twice and insist it must be the same both times. If it isn't
the same, the user has to type it over and over until the last two
times match.
The optional argument default specifies the default password to
return if the user enters empty input. If default is nil,
then read-passwd returns the null string in that case.
