Noticed while reviewing another patch that had an accidental
mismatch due to refactoring.  An audit of the code showed that
very few callers of vshCommandOpt were expecting a return of
-2, indicating programmer error, and of those that DID check,
they just propagated that status to yet another caller that
did not check.  Fix this by making the code blatantly warn
the programmer, rather than silently ignoring it and possibly
doing the wrong thing downstream.

I know that we frown on assert()/abort() inside libvirtd
(libraries should NEVER kill the program that linked them),
but as virsh is an app rather than the library, and as this
is not the first use of assert() in virsh, I think this
approach is okay.

* tools/virsh.h (vshCommandOpt): Drop declaration.
* tools/virsh.c (vshCommandOpt): Make static, and add a
parameter.  Abort on programmer errors rather than making callers
repeat that logic.
(vshCommandOptInt, vshCommandOptUInt, vshCommandOptUL)
(vshCommandOptString, vshCommandOptStringReq)
(vshCommandOptLongLong, vshCommandOptULongLong)
(vshCommandOptBool): Adjust callers.
Signed-off-by: NEric Blake <eblake@redhat.com>