run: add new --shell switch for spawning a shell as service

run: add new --shell switch for spawning a shell as service

I keep running "systemd-run -t /bin/bash" to quickly get a shell running
in service context. I suspect I am not the only one, hence let's add a
shortcut for it. While we are at it, let's make it smarter, and
automatically inherit the $SHELL of the invoking user as well as the
working directory, and let's imply --pty. --shell (or -S) is hence
equivalent to "-t -d $SHELL".