slirp: harden backend reinitialization

slirp: harden backend reinitialization

Ariadne can close and reopen the selected Ethernet backend while
guest software probes or retries card configuration. The SLIRP
backend destroyed callback state before the receive thread was known
to be stopped, and ignored initialization or thread start failures.

Join the receive thread before teardown, keep callback state alive
until shutdown completes, report SLIRP startup failures, and reset or
reuse BOOTP leases across backend restarts.