bus: add the ability for backends to queue to input messages at the same time

We need this so that one incoming kernel message can result in two
high-level bus messages, for the case where we synthesize NameAcquired
and NameOwnerChanged in the same instance.
bus: fix callback index when dispatching kernel messages
bus: synthesize NameLost/NameAcquired from kernel messages on top if NameOwnerChange
rename stdio-bridge to bus-proxyd
bus: revert attempts to make the integration-testing work

  fa6ccbf42360cdd794ae3164b76e0cc8f0bd62de
  197941a8d5d74d79ca45ae73ec114939a8e88244
  547708f59edd73d98edcbe84e95b41f7b3321f3b
build-sys: no longer rebuild various bits and pieces such as man pages each time the Makefile changes

We don't do this for .c files either, even they are also influence quite
a bit by makefile settings. Given that XSLT is a lot slower then the
rest of the build let's make our build a bit faster if people end up
touching the Makefile.
bus: don't generate NameAcquired/NameLost messages in the library

The signals appear entirely redundant due to NameOwnerChange, hence
don't do them in the new library, instead leave them for the
compatibility bridge only.

Also, set the serial number for synthesized messages to (uint32_t) -1.
bus: synthesize local error reply when we cannot deliver a message to kdbus because the destination is unavailable
bus: whenever we notice the connection is terminated enter CLOSING state
core: allocate a kdbus bus for each systemd instance, if we can
bus-proxyd: Fix CLI help. --bus-path is also usable as -p.
libsystemd-bus: catch up with latest kdbus changes

The name list and query functions now work differently in kdbus, as the
result is stored in the connection's pool, rather than being copied back
through the ioctl buffer. That also safes us some logic, as we don't
have to resize the communication buffer in a loop with growing buffer
sizes.

Due to this, KDBUS_CMD_MSG_RELEASE was renamed to KDBUS_CMD_FREE, as it
is now used outside the message logic as well.
bus: update kdbus.h
bus: include unique and well known names in credentials object
bus: append unique and well known names to all messages unconditionally
nspawn: set up a kdbus namespace when starting a container
libsystemd-bus: sync kdbus.h
bus: support temporarily const errors that don't need to be freed but require deep copies

This should fix issues with incorrectly copying bus error messages out
of sd_bus_message objects.

Original bug found by: Djalal Harouni
bus: use "System.Error." as prefix for implicit errno bus errors

We probably shouldn't use "Posix.Error" since many of the errors have
nothing to do with Posix.
update TODO