svc: use authuser for RealmInfo request
A user reported in this forum entry
https://sourceforge.net/p/sipe/discussion/688534/thread/
e7d7035e/#d3b1
that he gets a different response to the RealmInfo request if it uses
username or authuser. Only the latter response allows him to login
successfully.
Use authuser for the RealmInfo request, but only if it looks like
"user@domain". I.e. authuser will not be used if the user has specified
DOMAIN\account as login name.
(cherry picked from commit
0000548b61f7f91aa188f8d2945ca5bc3df96335)