vcf: new experimental bcf_hdr_set_samples() for subsetting

Parsing of VCF is slow. When only one or few samples are needed,
bcf_read() can be 4-5x faster for VCF after bcf_hdr_set_samples()
is called, however, the penalty of this for reading BCF is ~10%.

The performance was tested with
    bcftools query -f'[%SAMPLE\t%PL\n]' <file> -s <sample>
Merge branch 'bcftools+calling' into develop