ERP5Security,erp5: simplify EncryptedPasswordMixin.setPassword

For historical reasons, EncryptedPasswordMixin.setPassword was public
and did its own security checks, this was the case since 7d0882ef (
setPassword have to do explicit security checks…, 2007-11-12), this was
because we wanted to support cases where user can edit the login ("Edit
portal content" permission), but not changed the password ("Set own
password" permission).

Also, we wanted to support the case where login is edited through a view
form, in that case we have a my_password field that is empty and we
don't want to set the password to None in that case.

For these two reasons the API to set password was very complex and
behaving differently from other accessors: usually setSomething(None)
just set something to None, ie. "unset" something, but for passwords it
was not the case. Also we had to introduce _forceSetPassword method,
which sets the password without security checks, so that it can be
called from unrestricted code for cases where user does not have the
permission to reset password (like in the reset password scenario).

Since d1312cdb ( make edit check the security remove all useless
security declaration on private method, 2008-05-23), edit supports
restricted properties, so we can simplify all this and make setPassword
a more standard accessor, ie:
 - setPassword has a security declaration, so if it is called from
  restricted python the security will apply at `__getattr__` time.
  `edit` method will also check security
 - setPassword(None) reset the password.
 - The logic to not change the password when editing in view mode is now
  `edit` responsability. ie. `login.setPassword(None)` resets, but
  `login.edit(password=None)` does not reset.

This also correct some usage of the lower level API (`pw_encrypt` and
`pw_validate`) which were never supposed to use `None`:
 - `pw_validate` was called with None when a user without password was
  trying to login, causing a TypeError that was cached by PAS and logged
  with level debug (and refusing login). Now the error is no longer raised.
 - `pw_encrypt` was called with None (but apparently only in the tests,
  when doing `user.newContent(portal_type='ERP5 Login', password=None)`)
  and this was creating a login with password `'None'` with AccessControl 2.
  With AccessControl 4 this was an Error.