libudev: set errno properly in all error conditions of udev_device_new_from_syspath()
rfkill: rework how we generate file names from rfkill devices

Generate the file name from ID_PATH plus the rfkill type (wlan,
bluetooth, ...) and ignore the rfkill device name, since it apparently
is not a stable identifier.

Also, ensure that devices disappearing don't result in broken services,
simply exit cleanly.
machine-id-setup: Factorize some machine-id-setup functions to be reused
update TODO
machine-id-setup: casting const away is ugly, let's not do it if there's no reason to
machine-id-setup: add a machine_id_commit call to commit on disk a transient machine-id

If /etc was read only at boot time with an empty /etc/machine-id, the latter
will be mounted as a tmpfs and get reset at each boot. If the system becomes rw
later, this functionality enables to commit in a race-free manner the
transient machine-id to disk.
machine-id-commit: Introduce machine-id-commit binary

This binary enables to commit transient machine-id on disk if it becomes
writable.
machine-id-commit: add unit file

The unit file only active the machine-id-commit helper if /etc is mounted
writable and /etc/machine-id is an independant mount point (should be a tmpfs).
machine-id-commit: add man pages

Add man pages for systemd-machine-id-commit.service and
systemd-machine-id-commit.
build-sys: update man-list

Run 'make update-man-list'.
nspawn: correct EEXIST check when creating directory to mount /tmp in

https://bugs.freedesktop.org/show_bug.cgi?id=86309
localed: log xkbcommon errors

The errors are prefixed with "libxkbcommon" to provide some context,
because they are quite confusing without it. With the prefix, we at
least know where they come from.
strv: add calls to add two entries to an strv at once
udevd: don't take reference for NULL rtnl object
networkd: fix typo

V2: found another one
nss-myhostname: always resolve the host name "gateway" to the local default gateway

This is useful inside of containers or local networks to intrdouce a
stable name of the default gateway host (in case of containers usually
the host, in case of LANs usually local router).
rtnl: make checks for default routes more strict

Also check that the source netmask is 0, not only the destination
netmask.
resolved: don't resolve the hostname "gateway" with LLMNR, leave that to nss-myhostname
util: don't shadow variable

environ is already defined in unistd.h
networkctl: remove unused variable