Fixed handling of JOIN_TO registration events.

Calls io_detach for the old contig number.
Properly sets opt(io), opt(io2), opt(contig), opt(contig2) depending on
which contig(s) have been joined.


git-svn-id: https://staden.svn.sourceforge.net/svnroot/staden/staden/trunk@3231 ba6f2710-ca9f-427c-89d3-5bf716dbc8e1