Can't easily change DEFINER on existing views, stored routines, triggers, events

http://bugs.mysql.com/bug.php?id=73894

The question came up in the context of "DROP USER, how do I fix procedures afterwards?"

A naive expectation would be that one could simply say

  ALTER PROCEDURE name DEFINER=...;

but this isn't really supported on any of the four object types:

• ALTER EVENT can change the DEFINER, but only if at least one other
  attribute is changed at the same time

e.g. "ALTER DEFINER=foo@bar EVENT e1 COMMENT='';" works
but just "ALTER DEFINER=foo@bar EVENT e1" doesn't

• ALTER VIEW allows to change the definer, but you have to
  repeat the "AS SELECT ..." part

• ALTER PROCEDURE / FUNCTION doesn't support changing the
  definer at all, but you can at least work around it by
  changing it in the mysql.proc table and restarting the
  server afterwards

• ALTER TRIGGER ... well there is no ALTER TRIGGER statement
  at all to begin with ...

How to repeat:
Try to change the definer of existing EVENTs, VIEWs, PROCEDUREs, FUNCTIONs and TRIGGERs

Suggested fix:
Provide ALTER variants for all object types that have a DEFINER to easily change the definer with just

  ALTER $OBJECTTYPE $name DEFINER=...;