Add the header line to the output SAM

[nh13]

In particular, this defines the output SAM to be unsorted BUT also query grouped. The latter is very important to explicitly define so downstream tools that don't make assumptions know that reads from the same template are grouped.

[jmarshall]

If n_SQ is 0, this will print @SQ header lines before the @HD line.

(In fact this is really a pre-existing bug, as bwa mem -H '@HD\tVN:1.6' … would previously also have had the synthesised @SQ lines before the -H-supplied @HD header line.)