unshare: Add support for the pid and user namespaces

unshare: Add support for the pid and user namespaces

- Update the unshare application to support the pid and user namespaces.
- Update the man page for the new options
- Fix typo in the man page where UTS was spelled UTC.
- Remove the vestigal support for running a suid unshare.
  After unsharing a user namespace setuid(getuid()) won't work because
  no uid or gid mappings have been specified yet. So it is just easier not
  to have any support for running suid.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>