zfsd: add support for hotplugging spares

If you remove an unused spare and then reinsert it, zfsd will now online
it in all pools.

Do not MFC without 2a58b312b62 (but it's ok to MFC that one without this
one).

Submitted by: Ameer Hamza <ahamza@ixsystems.com> (zfsd), Me (tests)
MFC after: 2 weeks
MFC with: 2a58b312b62f908ec92311d1bd8536dbaeb8e55b
Sponsored by: iX Systems, Axcient
Pull Request: https://github.com/freebsd/freebsd-src/pull/697