MEDIUM: config: only warn if stats are attached to multi-process bind directives

MEDIUM: config: only warn if stats are attached to multi-process bind directives

Some users want to have a stats frontend with one line per process, but while
100% valid and safe, the config parser emits a warning. Relax this check to
ensure that the warning is only emitted if at least one of the listeners is
bound to multiple processes, or if the directive is placed in a backend called
from multiple processes (since in this case we don't know if it's safe).