spawning LFC does not inherit kea-dhcp environment

in memfile_lease_mgr.cc:

process_.reset(new ProcessSpawn(ProcessSpawn::ASYNC, executable, args));

in process_spawn.h:

ProcessSpawn(const SpawnMode mode,
             const std::string& executable,
             const ProcessArgs& args = ProcessArgs(),
             const ProcessEnvVars& vars = ProcessEnvVars(),
             const bool inherit_env = false);

so by default the lfc is spawned with empty environment

unless called explicitly by the system service with environment variables set, kea-dhcp spawns lfc with empty environment.

practical usage issues:

if using KEA_LOCKFILE_DIR to change the location of the logging lock file, kea-dhcp works properly, but lfc spawned by it does not inherit the value and uses the default path.