bash completions

[martinetd]

Just clush for now; might be some bugs but seems to work.

perf-wise it calls cluset -L on every tab, we might need to cache the result somewhere if it is slow in some setups. (But if user changes conf files it won't be reloaded so that's a bit annoying, it's better not to cache if it's reasonable e.g. just reading the conf files)

Prereq patch makes cluset -s foo -L not list foo groups with the @foo: prefix, like default source with -L -- this allows clush -s foo -w @bar to complete properly.