Prevent listen debug level to be forced under *BSD

Since 2.7.6 release, the standard error stream will receive many debug
messages. The problem happens when you have
`RbConfig::CONFIG['host_os']` matching `/bsd|dragonfly/`, there is no
way to disable this behavior and redirecting the standard error stream
is not a solution (when you use guard for example).

I understand why this was done, and I'm perfectly aware of
<https://github.com/guard/listen/issues/220>. However, even since
kqueue support is broken, polling still works and has always worked.
The change I suggest could allow some *BSD users (at least me ;-) to
continue to use polling with listen >~ 2.7.6.

I know *BSD platforms aren't supported, but I think this change won't
add any maintenance cost and will make life easier for some users
(maybe very few, I agree).