sd-bus: sync kdbus.h (ABI break)

In kdbus.h, the 'features' field has been dropped again. Instead of
negotiating features that way, we decided to make the kernel return the
set of supported flags in each ioctl struct's .flags field, in both the
success and error cases.
shell-completion/bash: add add-wants and add-requires
sd-bus: sync kdbus.h (ABI break)

kdbus.h now has KDBUS_ATTACH_COMM split into KDBUS_ATTACH_TID_COMM and
KDBUS_ATTACH_PID_COMM. The items were split already, so the change in
systemd is easy.
sysctl.d: default to fq_codel, fight bufferbloat

Quoting from Jon Corbet's report of Stephen Hemminger's talk at Linux
Plumbers Conference 2014 (https://lwn.net/Articles/616241/):

    [...] So Stephen encouraged everybody to run a command like:

    sysctl -w net.core.default_qdisc=fq_codel

    That will cause fq_codel to be used for all future connections
    [Qdiscs apply to interfaces, not connections. Pointed out by TomH
    in the article comments. -- mschmidt] (up to the next reboot).
    Unfortunately, the default queuing discipline cannot be changed,
    since it will certainly disturb some user's workload somewhere.

Let's have the recommended default in systemd.

Thanks to Dave Täht for advice and the summary at
https://lists.bufferbloat.net/pipermail/cerowrt-devel/2014-October/003701.html
man: document sd_machine_get_class() and sd_machine_get_ifindices()
man: suffix C functions with ()
man: document sd_bus_creds_get_connection_name()
update TODO
man: fix localectl set-x11-keymap syntax description
kdbus: free returned buffer when the memory is no longer needed
man: try to reword explanation of Sockets= a bit
Revert "bus-proxyd: improve compatibility with dbus-1"

This reverts commit b0f84d4d7832659f2216bda7a7cdf51f5e79c6eb.

get_creds_by_name() already translate the error nicely, we just need to
make use of it.
bus-proxy: let's make use of the translated errors get_creds_by_name() provides us with
sd-journal: consistently use ternary operator for all direction checks
update TODO
util: avoid duplication of TIME_T_MAX
shared: remove unused functions

- mkdir_p_prefix: It has never been used
- mkdir_parents_prefix_label: Unused since 1434ae6fd49f8377b0ddbd4c675736e0d3226ea6
man: use <example> instead of multiple <refsect1> for examples
man: move one more nspawn example into a proper <example> section
man: add missing commas