Upgrade problems

I attempted 5.x->6.x upgrade on a testing copy of my site, and got handful of errors. Attaching a patch.

- The existing single update function is a monster. Needs to be split, to finish without hitting PHP time limit.

- The attempt on set_time_limit() to unlimited won't work on many (most?) real sites, hosting accounts mostly run in safe mode.

- The code enables two modules dependent on privatemsg, without ensuring that privatemsg is enabled (6.x core run updates of disabled modules too!). Quite a few upgrade tutorials tell to disable contributed modules, then enable one-by-one. This way, we get into a state, where privatemsg is disabled, but still the checkbox on modules page is grayed-out due to dependency of (force-enabled) sub-modules.

- The code around transforming old folders into tags throws warnings, and imports all tags as empty-named. There are two problems: db_fetch_array / db_fetch_object mismatch, and extra space (syntax error) after COUNT in the query.

- Final cleanup attempts to drop 'privatemsg_block_user' table, even if that does not exist.

With the fixes (attached patch) the update works for me. But the tags part probably needs more changes (I believe this is being discussed somewhere else?), because the old folders were private (per user), and now a mix of all these is exposed to all users as tags. Plus, no old messages appear to be tagged according to their old folder-location?

As a sidenote, I'm not much excited about all the massive changes - to me it looks like turning a serious mail feature into a chatroom :-/ Especially that, when a message is sent, the message-writting form re-appears; that's like a call to spam people. But this is of course matter of taste. Seems like I need to delay upgrade of my real site yet another half a year, too.