bus: fix size calculation for match data
bus: do not fail if "comm" information cannot be retrieved from /proc
libsystemd-bus: factor out DBus bits out of sd_bus_get_owner()

Just a preparation for upcoming kdbus support.
libsystemd-bus: add sd_bus_translate_attach_flags()

Factor out code from sd_bus_negotiate_attach_creds() to a generic
translate function, so it can be used from other places.
libsystemd-bus: add kdbus support for sd_bus_get_owner()
libsystemd-bus: rename sd_bus_kernel_translate_request_name_flags

... to keep the namespace clean.
libsystemd-bus: sd_bus_request_name: use kdbus_translate_request_name_flags()