purple: fix icq login when the protocol name is "oscar"
Which happens when moving from non-purple to purple.
Fixes trac ticket 1269
Since "oscar" doesn't exist in purple, the old code called
register_protocol() to associate oscar with prpl-aim, which meant that
aim accounts migrated seamlessly to purple but icq accounts broke
silently, throwing incorrect password errors.
Now the oscar protocol is special-cased to return prpl-aim or prpl-icq
depending on the first character of the username, which is the same
thing the built-in oscar protocol does.
