sd-lldp: minor header cleanup

 * (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'