missing.h: add some IFLA_* constants

These were added to the kernel between 3.5 and 3.9, let's not require such
recent kernels (yet).
label: minor optimizations
update TODO
logind: remove redundant check in manager_new()
systemctl: remove erroneous return in runlevel_parse_argv()
networkd: link - only reset transient hostname if it was set by us
networkd: dhcp - log when lease is lost
sd-rtnl: test - include missing.h to build on old kernels
man: busctl typo fix
busctl: update bash completion
busctl: add --no-legend and use in bash completion
NEWS: Mention the new Personality= switch in unit files
make gcc shut up

If -flto is used then gcc will generate a lot more warnings than before,
among them a number of use-without-initialization warnings. Most of them
without are false positives, but let's make them go away, because it
doesn't really matter.
execute: modernizations
journal: don't clobber return parameters of sd_journal_get_cutoff_realtime_usec() on failure
sd-id128: use new dev_urandom() call
udev: make sure we always return a valid error code in trie_store()
timedatectl: fix minor memory leak
util: simplify get_ctty()
tmpfiles: simplification