inhibit: fix borked double logging on error

inhibit: fix borked double logging on error

Previously, if execution failed, we'd log at error level both from the
child and the parent, and we were using a bogus variable for the argument
name:
$ build/systemd-inhibit list
Failed to execute : No such file or directory
list failed with exit status 1.

In general, we can and should assume that the program the user is calling
is well behaved, so it'll log the error on its own if appropriate. So we
shouldn't log on "normal errors", but only if the child is terminated by
a signal.

And since the program name is controlled by the user, use quotes everywhere
to avoid ambiguity.

Now:
$ build/systemd-inhibit false
(nothing)
$ build/systemd-inhibit bash -c 'kill -SEGV $$'
src/basic/process-util.c:895: 'bash' terminated by signal SEGV.