Replace mkostemp+unlink with open(O_TMPFILE)

This will only work on Linux >= 3.11, and probably not on all
filesystems. Fallback code is provided.
journal: guarantee async-signal-safety in sd_journald_sendv

signal(7) provides a list of functions which may be called from a
signal handler. Other functions, which only call those functions and
don't access global memory and are reentrant are also safe.
sd_j_sendv was mostly OK, but would call mkostemp and writev in a
fallback path, which are unsafe.

Being able to call sd_j_sendv in a async-signal-safe way is important
because it allows it be used in signal handlers.

Safety is achieved by replacing mkostemp with open(O_TMPFILE) and an
open-coded writev replacement which uses write. Unfortunately,
O_TMPFILE is only available on kernels >= 3.11. When O_TMPFILE is
unavailable, an open-coded mkostemp is used.

https://bugzilla.gnome.org/show_bug.cgi?id=722889
manager: add systemd.show_status=auto mode

When set to auto, status will shown when the first ephemeral message
is shown (a job has been running for five seconds). Then until the
boot or shutdown ends, status messages will be shown.

No indication about the switch is done: I think it should be clear
for the user that first the cylon eye and the ephemeral messages appear,
and afterwards messages are displayed.

The initial arming of the event source was still wrong, but now should
really be fixed.
manager: also turn on output on unit failure