Added vcf_columns(); vcf_combine.c now strips sample info

1) global.c:vcf_columns() finds the nine standard VCF fields
2) Used vcf_columns to clean up code in vcf_combine.c and vcf_ref.c
3) vcf_combine.c strips genotype info from VCF
4) optimisation: merge_vcf_lines removed double call to vcf_columns()