sd-login: let's not needlessly yell at users

While GNOME/KDE are generally capitalized, systemd tools generally are
not, hence let's not start doing so in the XDG_CURRENT_SESSION
environment variable.
man: document that $XDG_SESSION_DESKTOP only takes a single item, not a list, unlike $XDG_CURRENT_DESKTOP
update TODO
udev hwdb: Support shipping pre-compiled database in system images

In some cases it is preferable to ship system images with a pre-generated
binary hwdb database, to avoid having to build it at runtime, avoid shipping
the source hwdb files, or avoid storing large binary files in /etc.

So if hwdb.bin does not exist in /etc/udev/, fall back to looking for it in
UDEVLIBEXECDIR. This keeps the possibility to add files to /etc/udev/hwdb.d/
and re-generating the database which trumps the one in /usr/lib.

Add a new --usr flag to "udevadm hwdb --update" which puts the database
into UDEVLIBEXECDIR.

Adjust systemd-udev-hwdb-update.service to not generate the file in /etc if we
already have it in /usr.
swap: replace Discard= setting by a more generic Options= setting

For now, it's systemd itself that parses the options string, but as soon
as util-linux' swapon can take the option string directly with -o we
should pass it on unmodified.
swap: drop noauto/nofail bools from Swap structure

We nowadays always set them to "false" anyway, hence let's get rid of
them entirely.
update TODO
mac: add mac_ prefix to distinguish origin security apis
man: minor corrections on how the overall system states are actually defined
update TODO
sd-bus: add support for new InteractiveAuthorizationRequired error from dbus spec
update TODO
polkit: actually generate new InteractiveAuthorizationRequired error on the right occasions
update NEWS
NEWS: add contributor list in preparation for 217 release
build-sys: bump library versions in preparation for 217 release
build-sys: bump version number in preparation for release
update NEWS
update NEWS
NEWS