libsystemd-bus: kernel: add sd_bus_kernel_translate_request_name_flags

Flags used to request a name from kdbus are not identical to what DBus
and sd_bus use internally. Introduce a simple function to do the
translation for us. It's factored out to a separate function so the
dbus-driver instance can make use of it as well.
libsystemd-bus: sd_bus_request_name: fix return value for kdbus

kdbus returns -EALREADY if the requesting connection is already the
owner of a name, and -EEXIST if the name already exists and the
connection is not able to take it over.

Also, n->flags needs a translation as well to match the SD_BUS_* enum
values.
systemd-stdio-bridge: make it socket-activatable and usable as kdbus bridge

Augment systemd-stdio-bridge a bit to make it a 1:1 bridge from legacy
DBus clients to kdbus. In particular,

 * allow setting the bus path of the upstream bus as command line
   argument
 * use sd_listen_fds() for systemd's socket activation
 * omit calling sd_bus_negotiate_fds() when upstream bus is kdbus
 * reply to bus send errors with proper dbus error messages
 * treat -ECONNRESET as expected end-of-connection condition
dbus bridge service and socket files
dbus core service ... wip commit

driver

driver
sd_bus_kernel_translate_request_name_flags