* (potentially) public headers must reside in src/systemd/ (not in
src/libsystemd*)
* some private (not prefixed with sd_) functions moved from sd-lldp.h to
lldp-internal.h
* introduce lldp-util.h for the cleanup macro, as these should not be public
* rename the cleanup macro, we always name them _cleanup_foo_, never
_cleanup_sd_foo_
* mark some function arguments as 'const'
