qemu-nbd: changes towards enabling -Wshadow=local

Address all compiler complaints from -Wshadow in qemu-nbd.  Several
instances of 'int ret' became shadows when commit 4fbec260 added 'ret'
at a higher scope in main.  More interesting was the 'void *ret'
capturing the result of a pthread; where we were conceptually doing
'(void*)(intptr_t)EXIT_FAILURE != NULL' which just feels wrong (even
though it happens to compile correctly), so it was worth a better
cleanup.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-ID: <20230922205019.2755352-2-eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>