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