[Patches] messaging cleanups

[Stefan Metzmacher via samba-technical]

Hi,

here're some minor cleanup patches for the messaging / tevent glue.

These prepare my work on getting a generic impersonation via tevent
wrapper contexts, this will allow to safely hook async path based
functions into the VFS layer later. See
https://git.samba.org/?p=metze/samba/wip.git;a=shortlog;h=refs/heads/master4-impersonate

The patches are made by Ralph and myself, already reviewed by Volker.
I'll push them shortly...

metze