Fix hseek() return value test

The previous fseeko() returns 0 for success, while hseek() returns
the new stream offset.  Testing >= 0 works for both.  Fixes a bug
introduced by 6416ebf2717cbcb956692f2eeec33649a9b25ec2.
Make filename argument a *const* char pointer
Strict matching of strings in bam_str2flag
Merge branch 'bcftools+calling' into develop