A battle is growing over the condition of a county-owned Long Island nature preserve that some describe as neglected, unsafe and in need of urgent attention. The sign leading into Brookside Preserve, a 20-acre park in Freeport owned by Nassau County, states the land is intended “for the enjoyment of nature.” But it has fallen into disrepair and become an eye sore — not to mention a public safety hazard, locals say, with garbage, broken walking p…