Revert "remote-fs.target: want remote-fs-pre.target"

This reverts commit 6bde0b3220e95a08cefb70846f73b2cf24b7734a.

We should not pull in remote-fs-pre.target unconditionally. It's
supposed to be pulled in by the implementors of it, rather then its
users.
units: introduce remote-fs-setup.target to pull in dependencies from remote mounts

This introduces remote-fs-setup.target independently of
remote-fs-pre.target. The former is only for pulling things in, the
latter only for ordering.

The new semantics:

remote-fs-setup.target: is pulled in automatically by all remote mounts.
Shall be used to pull in other units that want to run when at least one
remote mount is set up. Is not ordered against the actual mount units,
in order to allow activation of its dependencies even 'a posteriori',
i.e. when a mount is established outside of systemd and is only picked
up by it.

remote-fs-pre.target: needs to be pulled in automatically by the
implementing service, is otherwise not part of the initial transaction.
This is ordered before all remote mount units.

A service that wants to be pulled in and run before all remote mounts
should hence have:

a) WantedBy=remote-fs-setup.target -- so that it is pulled in

b) Wants=remote-fs-pre.target + Before=remote-fs-pre.target -- so that
   it is ordered before the mount point, normally.
update TODO
units: there is no point in pulling in ordering 'provides'-style targets

Units such as nss-lookup.target, nss-user-lookup.target,
remote-fs-pre.target, local-fs-pre.target, time-sync.target,
rpcbind.target are to be pulled in by the implementing services, and
that's there only purpose. They should not have any 'active component'
otherwise, so let's drop all further deps from these units.
man: rearrange systemd.special(7) to clarify which units are passive and which ones aren't

This also adds a short explanation paragraph for this.
man: properly document the system targets that are also available for the user instance