Add --map-user --map-group --map-uid and --map-gid options to restore.

Note that the usual metadata rules still appply, so a user or group
entry will normally take precedence over a uid/gid unless
--numeric-ids is specified.  So if you want to map a uid, for example,
you'll either need --numeric-ids, or you'll need to clear the
user/group like this:

  bup restore ... --map-user rlb= --map-uid 1000=2000

These options should also make it possible to recover from archives
that were broken as a result of our incorrect handling of
signed/unsigned stat values (recently fixed).

Signed-off-by: Rob Browning <rlb@defaultvalue.org>