user units: implicitly enable PrivateUsers= when sandboxing options are set

user units: implicitly enable PrivateUsers= when sandboxing options are set

Enabling these options when not running as root requires a user
namespace, so implicitly enable PrivateUsers=.
This has a side effect as it changes which users are visible to the unit.
However until now these options did not work at all for user units, and
in practice just a handful of user units in Fedora, Debian and Ubuntu
mistakenly used them (and they have been all fixed since).

This fixes the long-standing confusing issue that the user and system
units take the same options but the behaviour is wildly (and sometimes
silently) different depending on which is which, with user units
requiring manually specifiying PrivateUsers= in order for sandboxing
options to actually work and not be silently ignored.