mod_privilege fake session

[Nicoco Kinlidec]

This patch makes mod_privilege use a fake session to send stanzas on
behalf of a user.

This was discussed in prosody's MUC today. Let me know if I should
change something to get it accepted.

Cheers,

-- nicoco

[Kim Alvefur]

Merged.

I'm not familiar with this module so all I can say in the way of review
is that the luacheck warning count goes up.

--
Regards,
Zash