Split combined fields (bin_mq_nl and flag_nc) into their component
parts.

Move BIN calculation and Auxiliary tag encoding out of footnotes
and into their own sections.

The magic number is changed to "BAM\2" to allow software to detect
the new version.

The size of `n_cigar_op` is increased to uint32_t.

An extra field `bam2_hdr_flags` is added to the header, and
`bam2_flags` is added to the alignment records.  Both of these fields
are reserved for future enhancements.

The obsolete `bin` field is removed from version 2 files.

A new `BV` tag is added to the SAM `@HD` line as a (rather ugly)
way of hinting which BAM version should be used.