machine: validate machine names using machine_name_is_valid() instead of string_is_safe()

After all, we know have this as generic validator, so let's be correct
and use it wherver applicable.
cryptsetup: fix an OOM check
journalctl: add new --flush command and make use of it in systemd-journal-flush.service

This new command will ask the journal daemon to flush all log data
stored in /run to /var, and wait for it to complete. This is useful, so
that in case of Storage=persistent we can order systemd-tmpfiles-setup
afterwards, to ensure any possibly newly created directory in /var/log
gets proper access mode and owners.
update TODO
update TODO
units: run firstboot before sysusers, so that firstboot can initialize the root password
man: make udev.event-timeout more visible

Evidently some people had trouble finding it in the documentation.
sd-daemon,man: ignore missing $WATCHDOG_PID

Systemd 209 started setting $WATCHDOG_PID, and sd-daemon watch was
modified to check for this variable. This means that
sd_watchdog_enabled() stopped working with previous versions of
systemd. But sd-event is a public library and API and we must keep it
working even when a program compiled with a newer version of the
libary is used on a system running an older version of the manager.

getenv() and unsetenv() are fairly expensive calls, so optimize
sd_watchdog_enabled() by not calling them when unnecessary.

man: centralize the description of $WATCHDOG_PID and $WATCHDOG_USEC in
the sd_watchdog_enabled manpage. It is better not to repeat the same
stuff in two places.
journal-upload: fix socket activation
journal-remote: give names to event sources

This possibility was recently added, and it makes debugging much nicer.
systemd-upload: print paths in help()
journal-upload: do not require port to be set
journal-remote: better error message on failure

Return a proper code instead of simply NULL for failure.
journal-remote: add --split-mode to help
socket-util: use IP address when hostname is not found

socknameinfo_pretty() would fail for addresses without reverse DNS,
but we do not want that to happen.
journal-upload: verify state file can be saved before uploading

Do our best verify that we can actually write the state file
before upload commences to avoid duplicate messages on the server.
journal-upload: avoid calling printf with maximum precision

Precision of INT_MAX does not work as I expected it to.

https://bugzilla.redhat.com/show_bug.cgi?id=1154334
journal-upload: fix --trust=all option
shared/log: add log_trace as compile-time optional debugging

Repetetive messages can be annoying when running with
SYSTEMD_LOG_LEVEL=debug, but they are sometimes very useful
when debugging problems. Add log_trace which is like log_debug
but becomes a noop unless LOG_TRACE is defined during compilation.
This makes it easy to enable very verbose logging for a subset
of programs when compiling from source.
journal-upload: return proper exit code

Even when termninated normally, systemd-journal-upload would return
something positive which would be interpreted as failure.