Switch ping action from operating on explicit URIs to host components.

Also, remove the 'up' background thread with hardcoded pinging of hosts
from status://config/host/*.

Tweaked status.set to return the value being set, this is conventient when
using set as a deferred callback.
Fix a problem with some wildcard expressions.

Sufficiently nested wildcard URLs could raise a BadUrl in addition to
an UnknownUrl exception. In both cases, the exception just meant that
wildcard expansion was invalid.
Adjust the ping action to use wildcards, and ping multiple hosts.

It's now possible to request pings from more than one host at a time.