The opt_for_user subrule was incorrectly scanned before sp_create_assignment_lex(),
so the user name and the host were created on a wrong memory root.

- Reoganizing the grammar to make sure that sp_create_assignment_lex()
  is called immediately after PASSWORD_SYM is scanned, so all attributes
  are then allocated on its memory root.

- Moving the semantic code as methods to LEX, so the grammar looks as simple as possible.

- Changing text_or_password to be of the data type USER_AUTH*.
  As a side effect, the LEX::definer member is now not used when processing
  the SET PASSWORD statement. Everything is done using Bison's stack.

The bug sas introduced by this commit:
commit bf5a144e