When making a connection a "real" backend (VBE), we checked the probed
health state and did not take into account the administrative health
state as set with `varnishadm backend.set_health ... {healthy,sick}`.

Our documentation was not particularly explicit on this aspect either,
but the administrative states `sick` and `healthy` made no sense if
`auto` semantics was implied always. Also, the semantics were implicitly
documented for `backend.list`.

Implementation note:

The relevant change is to call `VRT_Healthy()`, which does check the
administrative health, in place of checking `(struct backend *)->sick`
in `vbe_dir_getfd()`.

As a `VRT_CTX` is required by `VRT_Healthy()`, we change the arguments of
`vbe_dir_getfd()` accordingly: The busyobj can now be taken from the ctx,
but the worker argument differs for pipe mode vs. fetch, so we preserve
it as an explicit argument.

A test for overriding a probed backend as healthy has been added to
c00048.vtc, which requires a second probe to hit server s1 and fail.
This is timing sensitive, so I hope that the backend probe interval
of 5 seconds is long enough for all our test environments. If not,
we probably need to make it longer or add another vtc.

Fixes #3299