Quoting usernames for Tramp

[Marcus Harnisch]

Hi all

My cloud provider uses an email address as username, so that it now 
contains two “@” (at) signs. Login fails and the reason appears to be 
that Tramp sees only the part up to the first “@” as user name. I tried 
escaping the username using “<name>%40<maildomain>@<server>”, but that 
didn't seem to help.

Any suggestions?

Thank you,
Marcus