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