userdbctl: also show available UID range in current userns

userdbctl: also show available UID range in current userns

Containers generally have a smaller UID range assigned than host
systems. Let's visualize this in the user/group tables. We insert
markers for unavailable regions. This way display is identical to status
quo ante on host systems, but in containers unavailable ranges will be
shown as that.

And while we are at it, also hide well-known UID ranges when they are
outside of userns uid_map range. This is mostly about the "container"
range. It's pointless showing the cotnainer range (i.e. a range UID >
65535) if that range isn#t available in the container anyway.