cgroup-util: Handle capsule@ paths like user@ paths (#36664)

The capsule instances are related to user instances, so treat them
equally to user@.service when handling cgroup paths. This also saves us
from polluting public libsystemd API with variant for capsules too.

Fix: https://github.com/systemd/systemd/issues/36098

src/shared/meson.build

@@ -148,7 +148,6 @@ shared_sources = files(
         'polkit-agent.c',
         'portable-util.c',
         'pretty-print.c',
-        'capsule-util.c',
         'ptyfwd.c',
         'qrcode-util.c',
         'quota-util.c',