sd-pppoe: include ppp_defs.h

On older kernels before this patch:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=e8b671460410c8fd996c8a1c228b718c547cc236
ppp-ioctl.h did not pull in ppp_defs.h which results in build errors
man: glib-event-glue.c - remove unnecessary includes

This is just an example, so no error-handling is done here anyway.
man: sd_event_add_post - fix typo
journald: constify all things!
journald: remove a number of malloc()s from the syslog message handling
journald: if available pull audit messages from the kernel into journal logs
core: introduce ConditionSecurity=audit

And conditionalize journald audit support with it
journald: fix minor memory leak
units: make ReceiveBuffer= line more readable by using M suffix
man: document audit input for journald
journalctl: add new --vacuum-size= and --vacuum-time= commands to clean up journal files based on a size/time limit

This is equivalent to the effect of SystemMaxUse= and RetentionSec=,
however can be invoked directly instead of implicitly.
journald: also check journal file size to deduce if it is empty
journald: fix memory leak on error path
update TODO
journald: there's no point in turning on SO_TIMESTAMP for audit sockets, audit doesn't support timestamps anyway
journald: enable audit in the kernel when initializing

Similar to auditd actually turn on auditing as we are starting. This way
we can operate entirely without auditd around.
journald: properly decode audit's proctitle= field
journald: suppress low-level audit text prefix in MESSAGE= field

Let's make the log output more readable, and the header can be
reconstructed in full from the other fields
journal: when dumping log data with missing COMM fields, show "unknown" instead

A small readability improvement...
journald: don't pass around SO_TIMESTAMP timestamp for audit, which we don't have anyway