This adds support for a new environment variable
SYSTEMCTL_INSTALL_CLIENT_SIDE, that ensures that systemctl executes
install operations client-side instead of passing them to PID1. This is
useful in debugging situations, but even beyond that. However, we don't
want to make it official API, hence let's just make it an undocumented
environment variable.

Similar, add a second variable, SYSTEMCTL_SKIP_SYSV which allows
skipping the SysV chkconfig fall-back if set. This is useful for similar
reasons, and exposed as undocumented as environment variable for similar
reasons, too.

When rebooting nspawn containers about 400 times we'd otherwise hit the
fd limit and refuse further reboots.

Let's make sure we don't even try to create the audit socket

- Make sure we log each error at least once, and at most once

- Replace FOREACH_WORD loops by extract_first_word() loops

- Use FOREACH_DIRENT() for directory loops

- Use free_and_strdup() where appropriate

- Do not operate on half-loaded SysV files

- Always properly free all memory

The default of 16 is pretty low, let's bump this to accomodate for more
queued datagrams. This is useful for AF_UNIX/SOCK_DGRAM logging and
sd_notify() sockets as this allows queuing more datagrams before things
start to block, thus improving parallelization and logging performance.

Let's make sure we don't start blocking on sd_notify() earlier than
necessary, let's bump the socket buffer sizes to 8M.

We already do something similar for our logging socket buffers, hence
apply a similar bump here.

Better generate a real error then simply connect to the wrong socket.

Otherwise we might run into deadlocks, when journald blocks on the
notify socket on PID 1, and PID 1 blocks on IPC to dbus-daemon and
dbus-daemon blocks on logging to journald. Break this cycle by making
sure that journald never ever blocks on PID 1.

Note that this change disables support for event loop watchdog support,
as these messages are sent in blocking style by sd-event. That should
not be a big loss though, as people reported frequent problems with the
watchdog hitting journald on excessively slow IO.

Fixes: systemd#1505.

…GNUTLS.