Amended/removed comments about scram_* API.
Merge short read() bug fix from upstream

See attractivechaos/klib@8d8d1a19f0c69b53d5ed8d9f6592dfa4b91c23f3
and lh3/seqtk#43.
Fix ks_getuntil2() extra empty record at EOF bug

When the stream is an exact multiple of the buffer size, ks_getuntil2()
was returning a final empty record when it should have returned -1.
Fixed by moving the "EOF => return -1" check to after the read loop.

Fixes samtools/samtools#318.  See upstream PR attractivechaos/klib#39.
bcf_hdr_subset: Return NULL on duplicate sample names
Sanity check to detect broken GT fields in bcf_calc_ac()
Add seq_nt16_int[], equivalent to the old API's bam_nt16_nt4_table[]

Also improve descriptions of the usage of all three tables.