build-sys: fix help text for --enable-kdbus
libsystemd-bus: the same error codes for sd_bus_release_name() (for kdbus and dbus1)

Due to this patch, sd_bus_release_name() function
returns the same code errors for kdbus and dbus1
if we try release non-existing name or foreign
name.
man: document more error codes for sd_bus_request_name()