view, new feature Major allele count filter

Added the ability to filter by 'major' and 'nonmajor' allele counts/frequencies in bcftools view.
It done by adding new allele types to the one the -c/C and -q/Q options understand.

Briefly, `major` is the most frequent allele (of all alleles, including the REF), and `nonmajor` is the sum of all but the major allele.
These (primarily `nonmajor`) is useful for filtering for sites which vary between samples.  All the other 'types' (like `nref` or `minor`) have issues with non-biallelic sites or sites were all (or nearly all) samples differ from REF.  In fact, I would argue that `nonmajor` is really what most people think of as `minor` when doing a min MAF filter.