A Single-Center, Randomized, Double-Blind, Placebo-Controlled, Six-Month Clinical Trial Followed by an Open-Label Extension to Evaluate the Safety, Tolerability and Clinical Endpoint Responsiveness of the Phosphodiesterase Type 4 (PDE4) Inhibitor Ibudilast (MN-166) in Subjects with Amyotrophic Lateral Sclerosis (ALS) - STEP-IBUDILAST-ALS-DB-OLE-1 (I8-1B)