unshare: Support multiple ID ranges for user and group maps

unshare: Support multiple ID ranges for user and group maps

Accept multiple --map-users and --map-groups options to specify disjoint ID
ranges to map inside a user namespace. This was already supported by mount
--map-groups and --map-users, but paradoxically not by unshare itself.

Factor out the hole-punching logic which removes single-ID mappings set
with --map-user and --map-group from range mappings set with --map-users
and --map-groups. add_single_map_range() now performs this fixup for
every given range before prepending the single-ID mapping.

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