Fixed SAM parsing code to handle integer values from -2billion to

+4billion, for proper BAM generation.

Fixed BAM to SAM conversion to correctly track whether the bam aux
item is signed or unsigned.  To do this the old bam_aux_iter has been
updated to become bam_aux_iter_full (the old one being a wrapper
around the new).  The full version also now returns the original bam
format code instead of the generic sam 'i' code for all integers.


git-svn-id: svn://svn.code.sf.net/p/staden/code/io_lib@3771 ba6f2710-ca9f-427c-89d3-5bf716dbc8e1