Implemented peer-review suggestions.

- Obviated need for bounds checking by building up a slice instead of tearing one down.
- Behavior of service filtering as it was before new filtering infrastructure.

Conflicts:
	apiserver/client/status.go
	cmd/juju/status_test.go
Merge pull request #1255 from katco-/v1.21

Backport: Implemented peer-review suggestions.

- Obviated need for bounds checking by building up a slice instead of tearing one down.
- Behavior of service filtering as it was before new filtering infrastructure.

Conflicts:
	apiserver/client/status.go
	cmd/juju/status_test.go

(Review request: http://reviews.vapour.ws/r/569/)