unshare: Don't waste an ID when -r is used with --map-auto

unshare: Don't waste an ID when -r is used with --map-auto

When --map-root-user or --map-current-user are used with --map-auto, one of
the IDs from the first range in /etc/subuid and /etc/subgid is wasted and
left unmapped:

  $ cat /etc/subuid
  1000:65536:100
  $ unshare --map-auto cat /proc/self/uid_map
           0      65536        100
  $ unshare --map-root-user --map-auto cat /proc/self/uid_map
           0       1000          1
           1      65536         99

In the second unshare, only 99 of the 100 UIDs available from /etc/subuid
are actually mapped, whereas in the first unshare, all 100 delegated UIDs
are correctly mapped.

Distinguish auto mappings from manually-specified ones so they can be
handled correctly, while still ensuring explicit --map-users/groups that
overlap with the single mapping are correctly reduced in length because
of the hole that's punched:

  $ unshare --map-auto cat /proc/self/uid_map
           0      65536        100
  $ unshare  --map-root-user --map-auto cat /proc/self/uid_map
           0       1000          1
           1      65536        100

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