bump ABI version

The ABI version was changed many times since the first release, but no
one noticed.  Now that I am packaging a newer version of libiscsi
I need to bump the soname from 1 to 2, which will be (posthumously)
the ABI for 1.9.0.  Declare the next version to have ABI version 3.

Please do not hesitate to ask for review of header-file patches to
catch ABI changes (which should be avoided!).