Implement view & bam2fq directly against the htslib API

Use the htslib names for accessors and I/O functions, and use an
iterator loop instead of bam_fetch and callbacks.

Equivalents to the old API's bam_get_library(), bam_remove_B(),
samfaipath(), and samthreads() are in progress and currently stubbed
out here.  (This temporarily causes one unused variable warning.)

Fixes #97 in passing.