core: accept "|" ExecStart= prefix to spawn target user's shell; teach run0 about...

I've always been reluctant to invoke the current user's shell in another
user's context, hence was fully grounded in `sudo -i`. With this bit in
place `run0` will finally be feature-complete on my side ;-)

diff --cc src/core/exec-invoke.c
Simple merge

diff --cc src/core/load-fragment.c
Simple merge

diff --cc src/shared/exec-util.c
Simple merge