namespace-util: make "setgroups" users property writable via userns_acquire()

namespace-util: make "setgroups" users property writable via userns_acquire()

Unprivileged namespaces are only allowed if the "setgroups" file is set
to "deny" for processes. And we need to write it before writing the
gidmap. Hence add a parameter for that.

Then, also patch all current users to actually enable this. The usecase
generally don't need it (because they don't care about unprivileged
userns), but it doesn't hurt to enable the concept anyway in all current
users (none of them actually runs complex userspace in them, but they
mostly use userns_acquire() for idmapped mounts and similar).

Let's anyway make this option explicit in the function call, to indicate
that the concept exists and is applied.