lvmdbusd: Correct lvm shell signal & child process handling

lvmdbusd: Correct lvm shell signal & child process handling

Previously when the __del__ method ran on LVMShellProxy we would blindly
call terminate().  This was a race condition as the underlying process
may/maynot be present.  When the process is still present the SIGTERM will
end up being seen by lvmdbusd too.  Re-work the code so that we
first try to wait for the child process to exit and only then if it hasn't
exited will we send it a SIGTERM.  We also ensure that when this is
executed we will briefly ignore a SIGTERM that arrives for the daemon.