bus: replace sd_bus_label_{escape,unescape}() by new sd_bus_path_{encode,decode}()

bus: replace sd_bus_label_{escape,unescape}() by new sd_bus_path_{encode,decode}()

The new calls work similarly, but enforce a that a common, fixed bus
path prefix is used.

This follows discussions with Simon McVittie on IRC that it should be a
good idea to make sure that people don't use the escaping applied here
too wildly as anything other than the last label of a bus path.