Read password, for use with KEY, from user, or from cache if wanted.
Then store the password in the cache. Uses `password-read' and
`password-cache-add'. Custom variables `password-cache' and
`password-cache-expiry' regulate cache behavior.
Warning: the password is cached without checking that it is
correct. It is better to check the password before caching. If
you must use this function, take care to check passwords and
remove incorrect ones from the cache.