[split] Add -d TAG option

[valeriuo]

Adds a new option to samtools split command, which allows the user to split reads into distinct files by a custom read tag (with a string type value). E.g. samtools split -d hc align.sam, where some of the reads in align.sam have a tag in the format hc:Z:<string value>.
Options -u is also available with -d.

Change the return type of auto_index method from char* to int, since a separate pointer to the index file name is no longer needed.

Depends on samtools/htslib#1055