ERCC1 and head and neck squamous cell carcinoma: Frequencies of XPC c.2815A>C, XPD c.934G>A, XPD c.2251A>C, XPF c.2505T>C and ERCC1 c.354C>T single nucleotide polymorphisms genotypes and haplotypes of head and neck squamous cell carcinoma patients stratified by response rate and toxicity to chemoradiotherapy