ikiwiki hosting internals/ bugs/ picks wrong username prefix if only one domain supported

Suppose you have this:

    # List supported domains here.
    domains=example.com

    # Username prefixes used to ensure unique usernames are used for
    # sites under different top level domains. Keep the prefixes short!
    prefix_ih=example.com

In `username()` we do this:

    # add unique prefix to avoid collisions with system users and
    # other top domains
    my $prefix;
    foreach my $key (keys %config) {
        next unless defined $config{$key};
        next unless $config{$key} eq $topdomain;
        $prefix=$key;
        $prefix=~s/^prefix_//;
        last;
    }

The value of `domains` is defined and equals `example.com`, so if it happens to come first in the hash (randomized order), ikisite will set `$prefix=domains`, leading to usernames like `domains-foo` instead of the intended `ih-foo`. At least in unstable, this happens inconsistently (again, randomized hash order) so I ended up with `foo.example.com` creating both `ih-foo` and `domains-foo`, and distributing its files arbitrarily between the two.

The solution is simple: `next unless $key =~ m/^prefix_/;`

Fixed in my `ready/prefix` branch: http://git.pseudorandom.co.uk/smcv/ikiwiki-hosting.git/shortlog/refs/heads/ready/prefix

--smcv

> fixed --Joey