switch roles: MDL-18132 separate database table role_allow_switch instead of re-using...

switch roles: MDL-18132 separate database table role_allow_switch instead of re-using role_allow_assign.

This is part 1 that does the back-end:

1. New table role_allow_switch.
2. Upgrade that copies all the allows from role_allow_assign, and then drops the old CFG->allowuserswitchrolestheycantassign.
3. Old function get_assignable_roles_for_switchrole renamed to get_switchable_roles and changed to use the new table. Fixes MDL-18604 in HEAD.
4. Switch callers to use the new function name.
5. Unit tests for this new function.
6. To make those unit tests work, new switch_global_user_id and revert_global_user_id methods in UnitTestCaseUsingDatabase for toggling $USER->id.

I still need to do the editing interface under Administration ► Users ► Permissions ► Define roles. That will be done as a second commit.