Implement purple_account_user_split_(is|set)_constant, remove unsupported Facebook XMPP features

Facebook XMPP: disable some unsupported features

Fix build release-2.x.y

Merge release-2.x.y

Facebook XMPP: perform roster cleanup only once release-2.x.y

Fill ChangeLog for Facebook XMPP roster fixes release-2.x.y

Merge release-2.x.y

Backport 68d162284480: facebook roster quirks release-2.x.y

XMPP: adjust facebook roster quirks

Merge release-2.x.y

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mercurial