Rewriting ("password obfuscation", WL#5706) now also works
for prepared statements, i.e. we try not to log any passwords
while handling prepared statements:

In the general log,
-- the "Query" line (for SQL PS) will no
   no longer contain the statement to prepare;
   if we prepare from a variable, its name is still
   given, but if we prepare from a string literal,
   that literal is not logged as we don't yet know
   whether it contains a password.

   - as always, --log_raw forces "old behaviour",
     i.e., the query is logged prior to parsing,
     and any passwords contained therein are printed
     to the log in plaintext

   - as before, PS protocol prepared statements
     only result in a Prepare line and an Execute
     line in the log (at prepare and at execute time,
     respectively); a Query line was not logged before,
     and is not logged now when PS protocol is used.

-- the "Prepare" line will contain the statement,
   with passwords obfuscated

-- the "Execute" line will contain the statement,
   with passwords obfuscated, and values filled in

Query:
prepare s4 from "set password=password('meow')";
execute s4;

General Log:
Query     PREPARE s4 FROM ...
Prepare   SET PASSWORD FOR `root`@`localhost`=<secret>
Query     execute s4
Execute   SET PASSWORD FOR `root`@`localhost`=<secret>


In binlog, passwords are replaced with their hashes (rather than
with a string literal) so replication still works. The
binlog line for the above SET PASSWORD statement is,

Binlog:
Query     use `test`; SET PASSWORD FOR 'root'@'localhost'='*82DC221D557298F6CE