Remove the 'list_open_transactions' register method

Before users were implemented, this was used at till startup to remind
staff about open transactions that might have been interrupted.  Now the
transaction each user is working on is recorded in the database and this
list isn't necessary.
The 'no sale' function is now controlled by a user permission
Fix description of priceguess function in tillconfig

We'd changed the calling convention in the Haymakers example but hadn't
updated the comment for the default function in tillconfig
Update department key handling
Add a permission for the 'drink in' key
Add some more permissions