DHFR and head and neck cancer: Association of the DNMT3B (-149C/T, -283T/C, -579G/T), DHFR 19-pb ins/del, SHMT1 1420C/T, and TYMS 28-bp Tandem Repeat Polymorphisms with the Primary Sites (Oral Cavity, Pharynx, and Larynx) in Head and Neck Cancer Group