BUG/MEDIUM: stats: never apply "unix-bind prefix" to the global stats socket

BUG/MEDIUM: stats: never apply "unix-bind prefix" to the global stats socket

The "unix-bind prefix" feature was made for explicit "bind" statements. Since
the stats socket was changed to use str2listener(), it implicitly inherited
from this feature. But both are defined in the global section, and we don't
want them to be position-dependant.

So let's make str2listener() explicitly not apply the unix-bind prefix to the
global stats frontend.

This only affects 1.5-dev so it does not need any backport.