Several improvements needed for other modules (forward, recipient_list, realname integration)

Ok, as proposed, these are some changes that were required to implemented the mentioned sub-modules.

If really necessary, I can split it up, but most changes aren't that big.

Changes introduced in this patch:
- a delete permission (we might need to add an info when upgrading that the permissions need to be updated)
- the username is properly themed in the reply form and view message list
- when replying a message, the recipient list is not re-parsed but instead read from the database.
- completely reworked _privatemsg_parse_userstring(). A hook is now executed that allows other modules to lookup a string. Note that the hook is only executed until a module does return a user object. If no hooks are available or if no user object is returned from any available hook, it falls back to the existing username lookup.
- a query_args key for select is added
- autocomplete usernames aren't validated before checked (necessary to get realname autocomplete working, as these might not be valid usernames)

If you have any questions, feel free to ask. This still needs some work (hook documentation, for example) but I wanted to upload it to get some reviews.