build-sys: add a link test for exported symbols

I know that this is a pretty big net to catch some small fish,
but we *do* regularly forget to properly export symbols that
were supposed to be exported.

This time sd_bus_get_current and some renamed symbols are caught.
build-sys: make public libraries depend on .sym files

This way modyfing the .sym file causes a rebuild.
systemd-python: fix booted() and add two functions to docs

For some reason sphinx doesn't want to show inherited C functions.
build-sys: parallelize 'exported' target