Reinstate bam_header_dup as bam_hdr_dup

In the original samtools API there was an undocumented call
bam_header_dup which was used to copy an existing header.  As I
found this is rather useful in certain situations and I have
reinstated this.

Signed-off-by: Martin O. Pollard <mp15@sanger.ac.uk>
Fix typo in comment

Signed-off-by: Martin O. Pollard <mp15@sanger.ac.uk>
Merge branch 'bcftools+calling' into develop
Make hfile.h part of the public API
Add MIT/Expat license boilerplate

As noted in the copyright notices, these files are recent additions and
have been contributed to solely by GRL.  So their provenance is clear.
Define __attribute__ macros in a new (semi-public) header