Fix virtuser_file plugin email2user() function not accounting for backward slashes in username

[idressos]

This addresses login issues when using aliases, if the resolved username includes a '@' character.
In the /etc/postfix/virtual file, usernames including a '@' character are saved with a leading '' character.

For example:
[email protected] becomes rocky.balboa**@**company.com

The function right above (user2email()) is accounting for this issue, it just seems it was forgotten for email2user().

I discovered this because I am using Roundcube with Webmin/Virtualmin.
Starting recent versions, Virtualmin includes the '@' character in internal usernames. In previous versions it was replaced by a '%' character, thus there was no need for a leading ''.

[pabzm]

Thank you for the fix!

Could you maybe add a short comment above the changed line to give future developers an idea, why this is necessary?

[idressos]

Done.

[pabzm]

@alecpl I'll wait one week for any comment from you, else I'd just merge this (since it's simple enough).

[alecpl]

Feel free to merge, don't forget to mention the change in the Changelog.

[pabzm]

@alecpl What about release-1.6? As far as I can tell you hand-pick the changes that go in there? Should this fix be backported, too?

[alecpl]

If it is a bugfix, and it is reasonably safe (small possibility to break something) and easy to backport then we should backport to release-1.6.

[pabzm]

So what's your conclusion regarding this PR? Backport it?

From your description I'd say yes, but since this has been your sole practice for a long time, you should decide.

[alecpl]

Yes.

[pabzm]

#9691