bus: introduce some sd-bus convenience helpers

bus: introduce some sd-bus convenience helpers

Many of the convenience functions from sd-bus operate on verbose sets
of discrete strings for destination/path/interface/member.

For most callers, destination/path/interface are uniform, and just the
member is distinct.

This commit introduces a new struct encapsulating the
destination/path/interface pointers called BusAddress, and wrapper
functions which take a BusAddress* instead of three strings, and just
pass the encapsulated strings on to the sd-bus convenience functions.

Future commits will update call sites to use these helpers throwing
out a bunch of repetitious destination/path/interface strings littered
throughout the codebase, replacing them with some appropriately named
static structs passed by pointer to these new helpers.