unshare: Add possibility to add mapping into root user in user namespace

This makes it very convenient to use make use of privileged actions
on CONFIG_USER_NS enabled kernels, without having to manually tinker
with uid_map and gid_map to obtain required credentials (as those
given upon unshare() vanish with call to execve() and lot of userspace
checks for euid==0 anyway).

Usage example:

$ unshare --uts
unshare: unshare failed: Operation not permitted

$ unshare --user --uts
[nfsnobody@odvarok ~]$ hostname swag
hostname: you must be root to change the host name

$ unshare -r --uts
[root@odvarok util-linux]# hostname swag
[root@odvarok util-linux]#

[kzak@redhat.com: - move code to map_id()
                  - use all-io.h
                  - add paths to pathnames.h]

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Karel Zak <kzak@redhat.com>