Add output mode parameter to bam_merge_core2 and bam_sort_core_ext
Add output mode parameter to bam_merge_core2 and bam_sort_core_ext

Instead of a compression level, these now take a sam_open()-style
mode string (e.g., "wb0" for uncompressed BAM).  This allows calling
code to ask for e.g. CRAM output.

mcshane/samtools,mcshane/samtools,mcshane/samtools,mcshane/samtools,mcshane/samtools,mcshane/samtools