core: Make DelegateNamespaces= work for user managers with CAP_SYS_ADMIN (#36771)

Currently DelegateNamespaces= only works for services spawned by the
system manager. User managers will always unshare the user namespace
first even if they're running with CAP_SYS_ADMIN.

Let's add support for DelegateNamespaces= for user managers if they're
running with CAP_SYS_ADMIN. By default, we'll still delegate all
namespaces
for user managers, but this can now be overridden by explicitly passing
DelegateNamespaces=.

If a user manager is running without CAP_SYS_ADMIN, the user manager is
still always unshared first just like before.