Updates to "checkfriends" protocol mode

[fu]

Working on some long-overdue updates to the checkfriends protocol mode.

Changes not yet live on the site but should be for future code push, but here's the run down:

• checkfriends no longer works. Instead, use checkforupdates.


• required arguments to checkforupdates is only authentication information


• optional argument lastupdate (in "0000-00-00 00:00:00" format). This is the last update time you have, from previous calls to checkforupdates


• optional argument filter. This is the name of a content filter whose members you want to filter to.


• return value: new: 1 or 0. 1 only if you pass in a lastupdate and there are new items since then. 0 in all other cases.


• return value: interval: number of seconds before you can next check for updates. If you check before time expires, you'll get a cached value


• return value: lastupdate: time someone last updated, in "0000-00-00 00:00:00" format

It's almost exactly the same as checkfriends; the only differences from the frontend are the name change, and replacing the mask argument with a filter argument because the trustmask is no longer relevant since we split up access and subscription.

ETA: Added preliminary documentation on the wiki.