Implements the change proposed in
http://sourceforge.net/p/samtools/mailman/message/30672431/

We use the bam_seq_t->bin field to hold the top 16 bits of the
cigar_len field when it is above 65535, also setting flags 0x8000 bit
to indicate this repurposing of bin.


git-svn-id: svn://svn.code.sf.net/p/staden/code/io_lib@3728 ba6f2710-ca9f-427c-89d3-5bf716dbc8e1