unit: do not order timers.target before basic.target

Since commit 19f8d037833f2 'timer: order OnCalendar units after
timer-sync.target if DefaultDependencies=no' timers might get a
dependency on time-sync.target, which does not really belong in early
boot. If ntp is enabled, time-sync.target might be delayed until a
network connection is established.

It turns out that majority of timer units found in the wild do not
need to be started in early boot. Out of the timer units available in
Fedora 21, only systemd-readahead-done.timer and mdadm-last-resort@.timer
should be started early, but they both have DefaultDependencies=no,
so are not part of timers.target anyway. All the rest look like they
will be fine with being started a bit later (and the majority even
much later, since they run daily or weekly).

Let timers.target be pulled in by basic.target, but without the
temporal dependency. This means timer units are started on a "best
effort" schedule.

https://bugzilla.redhat.com/show_bug.cgi?id=1158206
units: order sd-journal-flush after sd-remount-fs

Otherwise we could attempt to flush the journal while /var/log/ was
still ro, and silently skip journal flushing.

The way that errors in flushing are handled should still be changed to
be more transparent and robust.