Support SET PASSWORD for authentication plugins.
Authentication plugin API is extended with two optional methods:
* hash_password() is used to compute a password hash (or digest)
from the plain-text password. This digest will be stored in mysql.user
table
* preprocess_hash() is used to convert this digest into some memory
representation that can be later used to authenticate a user.
Build-in plugins convert the hash from hexadecimal or base64 to binary,
to avoid doing it on every authentication attempt.
Note a change in behavior: when loading privileges (on startup or on
FLUSH PRIVILEGES) an account with an unknown plugin was loaded with a
warning (e.g. "Plugin 'foo' is not loaded"). But such an account could
not be used for authentication until the plugin is installed. Now an
account like that will not be loaded at all (with a warning, still).
Indeed, without plugin's preprocess_hash() method the server cannot know
how to load an account. Thus, if a new authentication plugin is
installed run-time, one might need FLUSH PRIVILEGES to activate all
existing accounts that were using this new plugin.
Don't distinguish between a "password hash" and "authentication string"
anymore. Now both are stored in mysql.user.authentication_string, both
are handled identically internally. A "password hash" is just how some
particular plugins interpret authentication string.
Set mysql.user.plugin even if there is no password. The server will use
mysql_native_password plugin in these cases, let's make it expicit.
Remove LEX_USER::pwhash.
Cannot do password validation in fix_lex_user(), we don't know
there what "GRANT ... TO user" means - creating a new user with
an empty password (need validation) or granting privileges
to an existing user (no validation needed).
Move validation down into replace_user_table(). And copy into
check_change_password().
