bus-proxyd: handle -ESRCH and -ENXIO gracefully

Messages to destinations that are not currently owned by any bus connection
will cause kdbus related function to return with either -ENXIO or -ESRCH.

Such conditions should not make the proxyd terminate but send a sane
SD_BUS_ERROR_NAME_HAS_NO_OWNER error reply to the proxied connection.
bus-proxyd: ignore errors from sd_bus_creds_get_well_known_names()

sd_bus_creds_get_well_known_names() fails with -ENODATA in case the
message has no names attached, which is intended behavior if the
remote connection didn't own any names at the time of sending.

The function already deals with 'sender_names' being an empty strv,
so we can just continue in such cases.
core: rearrange code so that libsystemd/sd-bus/ does not include header files from core

Stuff in src/shared or src/libsystemd should *never* include code from
src/core or any of the tools, so don't do that here either. It's not OK!
env-util: don't include files from src/core/
import: fix compiler warning
condition: don't include files from src/core
sd-bus: teach x-container-unix: bus protoocol to connect to the namespace of a PID instead of a container name
machined: add new call OpenMachineLogin() that starts a getty in a container on a pty and returns the pty master fd to the client

This is a one-stop solution for "machinectl login", and should simplify
getting logins in containers.
machinectl: rework 'machinectl login' to use OpenMachineLogin()
update TODO
build-sys: move core/build.h → shared/build.h

After all, pretty much all our tools include it, and it should hence be
shared.

Also move sysfs-show.h from core/ to login/, since it has no point to
exist in core.
bus: add missing bus-policy.[ch]

Accidentally forgot to commit this. Sorry!
machined: introduce polkit for OpenLogin() call

This way "machinectl login" can be opened up to run without privileges.
gitignore: hide test-lldp files
busctl: when introspecting objects, optionally limit output by interface name
shell-completion: add full support for completing busctl command lines, with services, objects, interfaces, members, and signatures
update TODO
hwdb: Microsoft Wireless Laser Mouse 8000 DPI data

https://bugs.freedesktop.org/show_bug.cgi?id=87587
hwdb: add rule and first entry for PS/2 mice

https://bugs.freedesktop.org/show_bug.cgi?id=87037