Always call purple_privacy_check() for incoming IMs

[Mark Doliner]

How do people feel about the attached diff?  It changes serv_got_im()
to always call purple_privacy_check(), regardless of whether the PRPL
has implemented a set_permit_deny callback function.

It effectively does client-side blocking of incoming IMs regardless of
the protocol.  This has no effect for AIM, where the server enforces
the permit/deny setting.  But it benefits ICQ, where the permit/deny
setting controls invisibility and our block list is actually a list of
people to whom you're invisible.  Other protocols this might affect:
MSN, Yahoo, Novell, Sametime.

Based on Sulabh's email thread it seemed like people tended to be in
favor of this.

-Mark