core: remove "misuse" of getpgid() in systemd-shutdown

core: remove "misuse" of getpgid() in systemd-shutdown

Using `kill()`  with a signal of 0 is a slightly more documented idiom for
checking whether a process still exists.  It is mentioned explicitly in
man pages.  This avoids the need to comment the call as "misuse".
A comment is still necessary - in fact this idiom is even more confusing if
you don't know how it works.  But it's easy enough to explain.