unshare: Add --map-users=all and --map-groups=all

unshare: Add --map-users=all and --map-groups=all

Accept the special value 'all' for --map-users and --map-groups,
configuring a pass-through mapping of every user or group ID available
in the parent user namespace.

These are invaluable for scripting tests, for example when you want to
remap users leaving groups unchanged, remap groups leaving users unchanged,
or create a pass-through user namespace which maps the same users and
groups as its parent but has no capabilities in that parent namespace.

In the root user namespace, 'all' is equivalent to '0:0:4294967295', but
in child namespaces it may represent a fragmented mapping corresponding
to multiple --map-users or --map-groups options.

Signed-off-by: Chris Webb <chris@arachsys.com>