buildslave: don't fork when '--nodaemon' flag is passed

The '--nodaemon' flag was added in 2ae0ecb6347c2fd7b9301fe4dc3aaeebea235280
and efc3b6853ab7b03ba35de5820292a8bd7de96596 later fixed the
build master to not fork when this flag was used.  This commit
applies roughly the same fix to the buildslave.
Merge branch 'buildslave_noademon_fork' of git://github.com/andrewjcg/buildbot
release notes