Fix tbx_itr_next() when given a textual (is_kstream) htsFile

Previously to use a tabix index with a .vcf.gz file, you would open
the VCF_GZ with hts_open("rb") so as to have a BGZF*-based htsFile
(and remember not to use bcf_hdr_read() or bcf_read() with it!).
This doesn't work when the non-is_bin format is auto-detected.

Now tbx_itr_next() works with either kind of htsFile, though the kstream_t
is not currently kept synchronised, so you still shouldn't use bcf_read()
once you've started jumping around.  Eventually htsFile will probably stop
using kstream_t and this will be tidied up.

Thus bcf_sr_open_reader() no longer needs to reopen the file after
reading the header.