Updates for recent htslib changes (9dfcc31)

Use bgzf_raw_read/bgzf_raw_write instead of accessing underlying
knet/non-knet files (bam_cat.c, bam_reheader.c).  Knet is now
initialised in htslib/hfile_net.c (bamtk.c).  Samtools now does
not use knetfile.h directly or depend on _USE_KNETFILE.

Replaced "void *fp" by a union of the file pointers (sam.c).
Changed *_itr_next() to take htsFile*, not BGZF* (bam.h, bam_plcmd.c).
Need to include bgzf.h ourselves (bamshuf.c).