Patch: Non-unique msgid for abandon in back-<shell,tcl> (ITS#1793)

			================
Written by Hallvard B. Furuseth and placed into the public domain.
This software is not subject to any license of the University of Oslo.
			================

Here is a patch which does what I described.  Of course, someone has
to decide if that is the right solution:-)
- Add an "opid:" line to the input to back-shell commands.
- Add an "abandonid: <opid> line to back-shell/abandon input.
- Replace message id with opid in back-tcl arguments.
- Add an abandonid = <opid> argument to back-tcl/abandon.
An opid (operation ID) is a "connection ID/message ID" string.  I
would have liked to use another name to avoid confusion with struct
slap_op->o_opid, but I could not think of another apt word.

This also fixes ITS#1784 and ITS#1792.  Since calling conventions
changed anyway, I fixed back-shell by adding abandonid: and making
opid: always be the ID of the current operation.

Hallvard B. Furuseth <h.b.furuseth@usit.uio.no>, May 2002.