socket: make sure we warn loudly about symlinks we can't create

socket: make sure we warn loudly about symlinks we can't create

Note that this change does not make symlink creation failing fatal. I am
not entirely sure about whether it should be, but I am leaning towards
not making it fatal for two reasons: symlinks like this tend to be a
compatibility feature, and hence unlikely to be essential for operation,
in a way this breaks compatibility, and while doing that is not off the
table, we should probably avoid it if we are not entirely sure it's a
good thing.

Note that this also changes plain symlink() to symlink_idempotent() so
that existing symlinks with the right destination are nothing we log
about.

Fixes: #6920