log: fix fallbacks to kmsg

write_to_journal() returns 0 if journal_fd is closed and nothing is
written. We need to make sure we'll try log_open_kmsg() then to make the
fallback work for "journal-or-kmsg".
execute: robustness against journald failures

Almost every unit logs to the journal. If journald gets a permanent
failure, units would not be able to start (exit code 209/STDOUT).

Add a fallback to /dev/null to avoid making the system entirely
unusable in such a case.
execute: unify setup_{output,error}

The functions are quite similar. Unify them into one.

The source gets shorter, the binary gets slightly smaller.
execute: increase severity of journal connect failure message

journald is supposed to work. Failure to connect to its socket implies
losing messages. It should be a very unusual event. Log the failure with
LOG_CRIT.

Just because this unit's stdout/stderr failed to connect to the journal
does not necessarily mean that we shouldn't try to log the failure using
a structured entry, so let's use log_struct_unit.
unit: don't Require systemd-journald.socket from units

It is not really necessary to have a hard requirement dependency on
systemd-journald.socket in almost every unit. The socket gets pulled
into boot via at least two ways:
 sockets.target -> systemd-journald.socket
 sysinit.target -> systemd-journald.service -> systemd-journald.socket

So just assume something pulled the socket in and drop the automatic
requirement dependencies on it.

"systemctl stop systemd-journald.socket" will now not take the whole
system down with it.