fix remote fetching of regions for bam and vcf

hts.c: if `_USE_KNETFILE` is set, then remote fetching of a bam or vcf
region with `samtools view` or `bcftools view` will now work (after a
few attempts to get a usually non-existant `.csi` file).

fixes #69 (when `#define _USE_KNETFILE 1`, which it is not by default)